You have to know the signal before you leave the base. Before you EVER leave a base, find the baseball. This does not mean not to run hard, it just means don't run tense. Shoulders are level. Hips are level, like the shoulders.
Knees come up to the level of the hips.
The toes land before the heels. Run in a straight line. Good hitting mechanics allow a player to get a better jump out of the batter's box. First step out of the batter's box - Right handed hitter will take his first step to first base with his right foot. Last season, I saw a lot of kids get thrown out at first base because they were watching the ball instead of running hard.