Developing quick changes of direction, and quick reactions.
Players get into a good football stance about 2 to 3 yrds apart. The coach stands in front of them, holding a ball on the ground. The coach shouts out a cadence, and when he moves the ball they start moving their feet, watching the coach, and shuffling into the indicated direction, if the coach shows pass, they drop; if he shows run, they come forward. On command the players will sprint past the coach.
As a variation you can include seatrolls (only lateral movement), i.e. every change of direction begins with a seatroll to the new direction.
Instead of a command and the final sprint, the coach can drop the football and have the players recover the fumble, or throw a more or less interceptable pass and have them run it back.
Feet should always be moving and never cross on lateral movement. Players should always maintain a wide base. Quick change of direction is more important than speed on lateral movement.