Ice hockey skating

A skate blade is hollow-ground, which gives it an inside and an outside edge. Every push, turn and stop is made by setting one of those edges into the ice at an angle and loading it.

This is why a deeper knee bend produces more speed. Bending the knees lowers the centre of mass and lets the skater set the edge at a sharper angle, which gives the push something to work against. A skater standing upright has almost nothing to push from.

A hockey stop turns the skates perpendicular to the direction of travel and scrapes the edges across the ice. A tight turn does the same thing at a shallower angle — the edge bites, and the skater's momentum is redirected rather than destroyed.

Being able to stop in both directions is a genuine requirement rather than a refinement. A player who can only stop one way has to turn their body before they can stop, which costs the fraction of a second that decides most puck battles.

A crossover is the technique used to accelerate around a curve: the outside leg crosses over the inside one, and both legs push. Because two pushes happen per stride cycle instead of one, crossovers accelerate faster than straight-line striding.

This is why players so often take a curved route rather than a straight one when building speed from a standstill — the curve is not indirection, it is acceleration.