I use this as a warm-up, it helps players reacting and helping a team mate in case he make a mistake.
Have a group of 3 defenders on the field.
Coach throws a ball at one of the players and the closest player must run, touch that player's knee (the one who receives the ball) and return to his starting position.
Coach throws a ball to player 2, so the closest player (number 3) must run, touch player\rquote s 2 knee and return to his initial position.
Same if the ball goes to player 4.
Player 2 received the ball so player 3 (closest player) runs, touches player 2 knee and gets back.
If the Coach throws the ball to player 3, then the other two players must run, touch player's 3 knees and get back.
Player 2 and 4 return to their initial position.
You can have another group of players ready to replace these three players if the ball hits the ground.