You start off with a tennis ball and throw the ball continuously back and forth until somebody drops the ball.
When someone drops the ball you say "Down on one knee" and the person must drop down and continue play on one knee. If the same person drops it a second time then you say "Down on two knees". If the same person drops the ball again you say "Down on one elbow". If it happens again you say "down on two elbows".
It then goes to chin and then you're out but remember you have to stay in the position you're in to catch the ball and throw the ball.