As you can guess, this is not the cube form.

The Turret Cube, as you can guess, is a turret, diguised as a cube. Some levels have special legged sockets where you must place a turret cube. The only way to find one is if the heart on a companion cube opens up when you get close to it. If you pick it up, you have 5 seconds to place it before it fully awakes and blasts you to smithereens. When you place it down, a metal sphere encases it (To make sure it no longer blasts you to smithereens), and the wiring activates.