2-4 players improvise a scene according to any guideline. The rest of the players look on and hold themselves ready. Whenever a player feels overwhelmed, nothing more comes to mind, or he simply has no more desire to play, he shouts "Help!" and immediately another player comes from outside and replaces him.
This exercise is suitable for beginning improvisers, who take fear at staying in. The desired side effect is that the players play bravely, because they know at at any time they can go and another will always "get things underway" or "take the rap." So for example, one can always say something very daring/saucy and at once take off with a mischievous "Help!"
The same principle can be found in the game Stunt-Double.