For a specific level or just generally how it works?
It happens any time the lizard and light would otherwise end the move with a gap between them. For example, if you try to move while the light is stuck against a corner, the light can’t move so the lizard would end up stretching the chain an extra tile. This results in the light toggling and the player snapping back to the tile they started on. If you try the same setup where the light is stuck against the corner but something is blocking the player from moving (like a wall), that won’t cause a toggle because the lizard can’t travel far enough to form a gap. Let me know if there is a specific example where you’re curious why a toggle does/doesn’t work.