Pulling with the hamstrings, or striking in front of the COG is a bad thing for multiple reasons.
Stress on the hamstrings
breaking forces
Contact length, assuming optimal mobility and technique probably shouldnt be an emphasis ever. Teach a person to run this way and they will be injured and slower.
Increase stride length through sprinting and increase specific explosive hip extension. Decrease contact time through sprinting and improved specific reactive ability. Make sure mechanics are proper and optimal mobility. Sitting is bad, sprinting with a tall hip projection is best, as this is an indication that proper mechanics and vertical forces are in place.