The stage is divided in a left- and right side. Both sides are one location that are connected through the outside sides. The center of the stage is an imaginary wall through which the players can not walk, hear or see. The two locations do not necessarily have to have any relation to each other, as long as they do not require the characters to stay there for a very long time.
The four players play a free improvisation in two pairs. The left pair plays the left location and the right pair plays the same characters in the right location. If a player leaves the stage (always through the side) then their character enters the stage immediately on the other side. That character is then played by the other player, and they also find themselves in the other location.
You can play with this by for example not leaving the scene walking but rolling or jumping. The player on the other side copies this and also enters like this. When players shout something to the other location or throw something to the other they always aim for the sides.