In short: three players have to pass the ball to each other in a square. That's tricky, because one player is missing all the time...
Organisation: Three players stand in an imaginary square about eight metres from each other. One corner remains unoccupied. The middle player (number 2) of the three has the ball.
The aim is to play the ball along the lines of the imaginary square. Number 2 plays the ball either to number 1 or to number 3, for the sake of explanation: to number 3. Now the players have to make sure that number 3 also has two 'easy' passing points (along the lines of the square). In this case, number 1 must take care of that, she must sprint to the empty corner of the square as soon as she sees that number 3 receives the ball. Now it is number 3's turn to play (the situation is now as in the second figure). Suppose that she chooses number 1. Number 2 has to sprint to the place where number 1 just came from, so that number 1 has two points of play again. At the moment that number 1 has received the ball, the situation is (if number 2 has indeed run hard) as in Fig. 3. Suppose number 1 plays the ball back to number 3. Then it is number 2's turn to run again, etc.