What happens to the engine when the rear wheel is blocked?

The question may sound a bit strange, but the other day it had the discussion with a colleague and could not make us a rhyme on it, nor find anything in the net. So what happens to the engine when braking down without uncoupleing or switching the wheel (and thus the drive train) to block with the rear wheel brake? Is there a overload coupling (or does the “normal” then act as such)? Hope the question is not too stupid and I have brought it across intelligibly. Greetings!