This type of direction change, also known as “cross-under,” “compact or flexed transition”, “absorption”, or “retraction release”, features three distinct modes:
1. Top-down mode: The Center of Mass (CoM) moves toward the Center of Pressure (CoP).
2. Bottom-up mode: The CoP moves toward the CoM.
3. Hybrid mode: The CoP and CoM converge and meet halfway.
The edges are changed by flexing (absorbing by legs’ retraction) and the extension after is performed on the “new” edges. Body oscillation is performed not so much towards the center of the new turn (lateral oscillation) like in the Direction Change by Extension but diagonally.
In this direction change mode, the strategy is release-and-engage, this is, releasing the downhill and, at the same time, engaging the uphill ski while both legs are flexing (absorbing-retracting) simultaneously.
This mode of changing direction is mainly used in short turns, on bumps, deep snow, crud/crust snow, or in any other situation where, due to limited space and time, the curvilinear or linear trajectory must be changed quickly.
Although it is the fastest way to change direction, it tends to a loss of snow contact sensation (skis feel “floating”). Also, there is no economy of muscular effort due to the marked activation of the quadriceps and the location of the pelvis in moderate or marked position towards the tails of the skis in the Generation Phase (falling into a “backseat” position).
Therefore, we can say that it is effective but not efficient, however it results in increased stability, it is effective in tight spaces, and it gets better performance in dense snow.
Biomechanical Phases
1. Generation phase
In the bottom-up mode, the Center of Pressure (CoP) moves vertically toward the Center of Mass (CoM), generating active absorption of both knees and initiating the edge change, which is completed when the CoM crosses over the Cop.
In this phase, the skier breaks away from the previous curvilinear trajectory to begin a new trajectory in the opposite direction. The generating movement is the simultaneous flexion of both legs.
2. Monopodal Phase
The inner arch of the uphill foot (new standing foot) makes contact with the snow through foot pronation. The new leading foot remains lightly or moderately loaded on the edge of the 5th metatarsal (little toe edge), depending on snow conditions or skier’s intention.
3. Oscillation Phase
From the support on the new edges, the active extension of the uphill knee begins, mainly controlled by the hamstrings, and the passive extension of the downhill knee together with the forward movement of the pelvis.
In this phase, the pelvis continues its translation in the direction of the turn to generate and then maintain the Centripetal Posture. The edge change is simultaneous: both skis change edges at the same time.
4. Bipodal Phase
This is the longest phase of the turn, in which the support between both feet tends to level out until the cycle is completed and the Generation Phase is repeated in a new direction change.
Technical Framework Matrix for Direction Change by Flexion
| Terrain & Trajectory Constraints | Spatial Mechanics & CoM/CoP Proxemics | Biomechanical Phase & Muscle Execution | Edge State & Functional Terminology | Cognitive Load & Sensori-Motor Response |
| Short Turn Radii | Restrict space limits to trigger fast lateral trajectory alterations | Retract lower limbs rapidly to fit within abbreviated corridors | Concentric Turning: Tight execution mode optimizing line changes | Process extreme temporal compression without freezing movement |
| Moguls & Bumps | High-amplitude vertical profile challenges stability metrics | Absorb crest impacts via intentional dual knee collapse | Absorption: Dynamic knee tracking to neutralize terrain spikes | Anticipate rapid vertical displacement to stay ahead of the crest |
| Deep Snow Packs | High-resistance medium requiring continuous planning overrides | Maintain planing attitude while changing structural directions | Compact Transition: Parallel limb flexing inside dense powder fields | Overcome resistance drag by committing mass to the deep arc |
| Crud & Crust Snow | Variable surface friction profiles with high tracking hazards | Flex both legs simultaneously to slice through breaking crusts | Retraction Release: Unlocking edge platforms beneath heavy snow | Absorb unexpected high-frequency snow pack deflection shocks |
| Limited Space & Time | Minimal geometric margins demand hyper-fast redirection paths | Execute sudden, sharp direction change sequences on the fly | Cross-under: Direct underneath movement bypassing torso tipping | Execute immediate motor adjustments when exit lines disappear |
| Vertical Mass Approximation | Linear vertical trajectory axis changes down the pitch | Move the Center of Pressure vertically closer to Center of Mass | Concentric Turning: Compact vertical profile manipulation | Track compression limits to prevent absolute system collapse |
| Diagonal Oscillation Track | Deviation from standard lateral center-of-turn pathways | Displace the body mass diagonally rather than purely sideways | Diagonal Oscillation: Complex multi-planar mass translation | Calculate diagonal vector angles relative to incoming fall line |
| Simultaneous Limb Flexion | Dual-track parallel lower leg structural framework | Collapse both knees and ankles into deep flexion concurrently | Concentric Turning: Synchronized contraction of lower body segments | Coordinate dual-limb mechanics to prevent asymmetric tracking |
| Post-Transition Extension | New edge tracking platform configurations | Perform active limb extension only after new edges bite snow | Retraction Release: Delayed extension sequence across transitions | Suppress the urge to extend early before platform establishes |
| Downhill Edge Unlock | Old lower ski trajectory boundary lines | Relax downhill leg extensor muscles to initiate release phase | Release-and-Engage: Dual action edge transition strategy | Process sudden platform drop without losing upper body posture |
| Uphill Edge Bite | New upper ski trajectory boundary lines | Pronate uphill foot to engage big-toe edge into new arc | Release-and-Engage: Early uphill biting during deep flexion | Commit mass to the unweighted uphill side without hesitation |
| Floating Sensation Paradox | Loss of direct snow contact pressure signatures | Soften leg extension to let skis skim across transition zones | Retraction Release: Floating phase where track tracking drops | Manage panic responses when foot sole tactile feedback blurs |
| Quadriceps Over-Activation | Muscular endurance threshold constraints | Engage quadriceps eccentrically and concentrically under high loads | Concentric Turning: High-fatigue mode requiring maximum drive | Monitor local muscular fatigue to prevent breakdown over ruts |
| Pelvic Tail Placement | Backward mass distribution alignment anomalies | Position the pelvis moderately or markedly toward the ski tails | Backseat Stance: Non-efficient tail-weighted posture setup | Counteract tipping backward by driving hands and shins forward |
| Stability Optimization | High-speed structural equilibrium goals | Lower center of mass toward snow pack to maximize balance | Compact Transition: Ultra-stable low-profile posture configuration | Rely on low center of mass safety margins across icy slopes |
| Curvilinear Breakaway | Old curved trajectory separation boundaries | Snap away from previous arc to enter opposite directional path | Generation Phase: Active entry segment initiating track change | Focus visual field downstream to target the new arc path |
| CoM Cross-Over | Geometric transition interception points | Translate the CoM explicitly over the low CoP platform position | Generation Phase: Final crossing point completing edge swaps | Time the cross-over moment to align with terrain troughs |
| Uphill Arch Pronation | Big-toe edge anchor zone of new tracking foot | Drive inner arch of uphill foot downward via ankle pronation | Monopodal Phase: Single foot tracking sequence on new edge | Balance over the inside edge before secondary foot loads fully |
| Little-Toe Edge Loading | Little toe boundary of new leading foot | Apply light to moderate load onto the 5th metatarsal | Monopodal Phase: Stabilizing track control via lead foot edge | Adjust little-toe pressure dynamically based on snow density |
| Hamstring Extension Control | Posterior chain muscular firing paths | Extend uphill knee progressively under hamstring management | Oscillation Phase: Controlled limb lengthening across fall line | Rate-limit leg extension to match incoming centripetal load |
| Passive Limb Extension | Downhill tracking leg structural variations | Allow downhill knee to extend passively matching the terrain | Oscillation Phase: Asymmetric leg lengthening tracking path | Isolate passive leg adjustments from active core bracing loops |
| Forward Pelvic Drive | Sagittal mass translation corridors | Push pelvis forward continuously to exit the backseat stance | Oscillation Phase: Structural correction moving mass over feet | Force hips forward to re-engage ski shovels with the snow |
| Centripetal Posture Loop | Dynamic lateral inclination profiles | Maintain persistent lateral angulation to counter turn forces | Centripetal Posture: Angular body shape fighting mass inertia | Check angulation limits against maximum turn speed vectors |
| Simultaneous Edge Swap | Dual ski chassis alignment matrices | Change edge tracking angles on both skis at the exact same instant | Oscillation Phase: Zero-delay double platform rolling action | Synchronize foot roll to avoid scissor-tracking line errors |
| Foot Support Leveling | Symmetric double platform load distribution | Equalize pressure values between feet through the turn apex | Bipodal Phase: Main turn duration phase using dual skis | Maintain uniform tracking arcs by splitting forces |
