This is listening warm-up. Everyone start milling about the room.
Everyone quietly mills about the room. One person will elect to freeze in position unexpectedly. As soon as one notices that someone else has frozen in position they freeze as well. So the effect of one person freezing causes everyone to freeze. Once everyone is still the group starts milling around again. The goal is to see how quickly the group can freeze in position.
The warm-up can be made more interesting by having the players make noises as they move around. Increasingly noisy characters make it harder and harder to notice the group stop, and therefore makes it more challenging.