You will need 1 QB, 1 center and 3 Defensive backs. The center will have the ball at the 10 yard line. One DB will be lined up on the 5 yard line between the hash marks, with the other 2 DB's just outside the hashes on the 5 yard line.
On QB cadence the center will snap the ball (I prefer to run this out of the "gun"). The DB's will backpedal into the end zone at this time, with the QB taking a 3 - 5 step drop. The QB will try to throw the ball into the end zone and hit the ground without the defenders touching it. The defensive backs will react and close on the ball and attempt to knock it down or intercept it. If the ball hits the ground first, the QB earns 2 points. If the defender knocks it down, he gains 1 point. On an interception, the defense earns 2 points. Just for fun, you can have the losing "team" do a little extra "conditioning."
This is excellent practice for defensive backs to react to the ball once it is in the air. Watch for good fundamental drops by the DB's. All three DB's should "flip" their hips when reacting to the pass and close on the ball at full speed. My players have a tendency to try to predict the pass before it is thrown, so have your QB look off the defenders or pump fake them.
Excellent drill for keeping the feet "under control" and reacting to a thrown ball. Excellent agility builder as well.