This is a drill that I initially used for my secondary team but transitioned for use as conditioning drills for football for all the teams.
Your team should be aligned in groups of 6 or 7 (whatever evens your lines off).Two different colored cones should be placed opposite of each other about 15 yards from the groups.
On the whistle the 1st group should backpedal with proper form as fast as they can. The color of the cone you want them to attack is called followed by a quick whistle that does 2 things: tells them to sprint around the outside of the cone with each player urged on to catch the man in front, and to start the next group to backpedal.
The 2nd whistle should be blown at different times sometime after the group backpedals past the cones to keep the guys honest.