I just picked up the Graphical Player 2008 and I’m taking a look at the speed index for batters.  Problem is, I’m a little slow and I don’t understand it.
The description about the speed index says “Speed Index(SX) is just an estimate of the rate at which a hitter tries to steal (successfully or not).” It’s supposed to be a reflection of the manager’s tendency to send a player.  The numbers in the book range from zero to 600, with lines at 200 & 400. 
Can someone explain this in a way I can understand this?  For the life of me I just can’t figure out what the Speed Index means or how it’s calculated.  Thanks in advance!