Half of the cube’s edge pieces have small spring-loaded latches that press into the center piece, and when you twist a layer into place, these latches lock into the center piece, preventing you from turning the layer back. This mechanism makes the cube a bit slower to turn than a normal cube. When you turn a layer, it clicks when the latches snap into place. In return for the slightly slower cube, you get a tricky and fun challenge that requires more of you than the usual Rubik’s cube.
Below you see a picture of how the locking mechanism looks like in the core of the cube: