Grass Sprint Training

Basically it affects the way your leg responds. Because it is exerting more force to move quickly on a softer surface, when you return to the track (carefully over time) your legs are prepped for a softer surface and transfer force as if it were on grass into the less pliable surface of the track.

This is a poor explanation, I’ll try and dig up a Charlie Francis forum post on the subject which lays it out pretty well.

Also, supposedly it strengthens the legs more, might make you less prone to injury, same as with distance running (YMMV)

Here it is:

http://community.charliefrancis.com/showthread.php?16763-Effect-of-Training-on-Grass