It pretty much locks solid. If you imagine the shaft in the transmission tied to reverse that the driveshaft hooks to, and at the back of it, there is a large "gear" with big teeth in it, and a lock pawl that drops into the teeth of that gear and keeps the shaft from being able to turn, you have it down in it's basics. When there is throttle pressure, meaning the engine is running, that pressure holds the lock pawl away from the gear teeth.
