These are two sprint variations that I will use with my athletes when they are first learning how to sprint.
Often times coaches will have athletes start to low (or simulating like they are coming out of the blocks). However the problem with this is the athlete is often not strong enough to power out from this position to obtain the proper shin and torso angles for acceleration.
By using a momentum start or falling starts you can get the athletes into better positions for acceleration until their strength levels start to increase enough to go down into a 3 point or 4 point stance.