hahahaha wow this man http://media.crossfit.com/cf-video/C...ng_tw0_PRE.mov
Look you do not intentionally "pull your foot up" or intentionally "contract your hamstrings" to bring your foot up - this is how you tear your hamstring.
You don't "stop using the hamstring, because you have to" this is complete nonsense.
Bmac is teaching people how to tear their hamstrings.
In the support phase (middle row, 4th picture in the lagat picture above) you are absorbing impact, just like a box jump, and eccentric muscle action is happening to control this impact and the body sinking, it switches to concentric contraction for the push off and leg drive (middle row, 5th picture, and bottom row). The whole point of this leg drive and pushoff is to propel your center of mass forward (your hips). This is why runners have low vertical jumps and long horizontal jumps- because they're NOT vertical "jumpers", they're horizontal "jumpers"- GLASSMAN. When the leg drive ends into flight phase (top row, 4th & 5th picture), all that muscle tension in the concentric pushoff and leg drive is released into free air being its not fighting the ground anymore; this is why the faster you sprint, the more force generated, the further your leg flys behind you= tension release. THIS IS A REACTIONARY MOVEMENT, NOT VOLUNTARY. The hip flexors in this back leg drive and pushoff are stretched in an eccentric contraction (left leg, top row, 4th picture), in flight phase, this stretched eccentric tension is released, immediately turns concentric, and swings the thigh forward, the hamstrings eccentrically tense to act as stabilizers and control the ROM (this is NOT A VOLUNTARY CONTRACTION), shin folds under, the closer the shin folds into the body, the smaller the lever, smaller levers travel faster.
Bmac isn't teaching ANY OF THIS. He's just saying pull your heel up and lean, this is nonsense.