Everyone is right here :saint:. 2belite is right in saying that ground contact occurs in front of the COM. This is pretty much an inevitability of running when you're a human :(. However, everyone else who commented about minimizing the distance between the point of ground contact and the plumb line beneath the COM is also correct. In all, braking forces are inevitable but minimizing them is one of the biggest factors for improved performance in the sprints.
As for the rest of 2belite's comments, I'm a little divided. While I don't think sprint drills are a great way to develop sprint mechanics in and of themselves, they can provide a method of teaching specific cues about specific portions of the sprint cycle. Having said that, I think that a B movement (as I've described it above as opposed to the typical 'kick-out' version) can have some benefits beyond dynamic flexibility and physical preparation. I agree to some extent that the opening of the leg is a result of the rapid change of direction of the leg, this is why I tried to emphasize the point that it should occur as a result of the rapid downward acceleration of the leg rather than an active kickout. However, the opening of the leg is also a result of changes in muscle length and tension to the hamstring muscle as the hip is being extended. Consequently, the degree of "opening up" is related to two factors (the degree and rate of change of direction of the thigh, and the consequent changes in the muscle length and tension of the hamstring muscle group). As such, the opening up will still be present in a B drill but we'd expect it to be to a lesser extent than in actual sprinting.
ELITETRACK Founder