Before the Shaman Update the cheese on the the right is fake, though it can be hard to tell with many similar levels. If you are quick, you can touch the cheese on the far right and tell if it's fake or not. When you find out, run quickly back to the left. After this the trap appears at the same time the real cheese spawns to the right of the mouse hole.
Currently the cheese on the right is actually real and the cheese on the left is removed. Quickly move on and stop were the trap is near the bush. After 12 seconds a cage will spawn at the right, trapping the mice that went to the cheese instead of stopping at the rush. Many mice will push the cage to the right to kill the trapped mice because they were getting the cheese.
Formerly the shaman can help the mice by using the Arrow to the right of the mouse hole, which indicates where the real cheese spawns. However some mice, usually new players, venture to the right and get trapped in a cage. Some shamans prefer to push them into the pool of water, although if you choose to save the innocent mice, they'd be glad for it. With varying success, a shaman can push the mice out with a cannon ball, or fill the cage with balloons which sometimes breaks upon reaching the level limit, allowing the mice to drop back to the bottom. A very safe way to open the cage is by using the spirit glitch. It could take some time, but when done correctly all mice will be saved. (don't try this without practicing it first).
However, the shaman is removed to make it more difficult for mice.