"Butt kicks" should not be used to improve sprint mechanics because they do not reinforce good sprinting form. If you are sprinting with correct form your heel will never reach your butt with your thigh in line with your upper body. If it does, it means that you are recovering too high and wasting energry and time (lowering your turnover rate). Your recovery leg should not go far past parallel with the ground before you begin to drive your knee forward, and thus your heel should not come near your glutes. An excessively high leg recovery is probably the most common mistake made in running form, watch athletes such as Marion Jones and Mo Green and you will notice their recovery legs do not come near their glutes.