Thankfully someone explained how the anti-rollback devices on an Intamin Mega works on TPR a few days ago:
As the wheel spins against the lift-hill the magnet will lift the anti-rollback device lifts from the holes so that they don't make any sound, but if the train stops the magnetic field cant hold the magnet down which causes them to drop and lock in the holes on the lift hill.
Also when the train is moving slowly (at the start of the lift) the copper wheel isn't spinning fast enough to create the magnetic field to lift the anti-rollback devices, which causes them to clank a bit before you are on your way up the lift.
The magnetic stuff is somewhat similar to the braking action on a Magnetic brake...
Source: TPR