It's all about getting in motion, improving the reaction time and quickening the first 10 ft. as Steve Sylvester is correct in his answers. There are a few points of interest I like to share.
While many have their theory as to the proper technique, I have mine as well. I teach the "rock and run" to time the release. Ideally, I prefer they put their left foot onto the leading edge of the bag at the farthest corner from home plate, using that corner of the bag to push from. This technique has them stepping with their right foot first from the "outside" of the bag so there is no way the stride is impeded by the bag. It's also the farthest from home plate and should be clear of any fielder trying to move towards the bag. It also allows the first step of the runner to open up towards home plate to see what is going on. Some like to set up so they take their first step over the bag but I have seen them snag a toe on the bag as they initiate that first step. Some like to set up with the bag behind them and react like a track runner from a stationary position but there is a clear difference in the first 10 ft. in the reaction time if you "rock and run" as suggested.
Why they want to stand beside the bag with the push off or launch foot beside the bag touching it only with the side of the foot makes absolutely no sense to me. You don'y see and sprinters in track set up beside their starter blocks to run a race. Geez, some times being cute isn't practical. Just my opinion.
