If someone blocked it successfully the engine would stop because it can’t expel the exhaust gas on the second stroke of the piston.
if you only partially blocked it the blockage would be expelled. There is a lot of pressure on exhaust gas.
To see how much, put a rag over your hand and hold it over the exhaust pipe.
Mechanics use this as a quick test to see if the exhaust pipe has any leaks in it – if the engine continues to run then there is a leak else the engine starts to sputter and cut out the exhaust is okay.
Do it and you’ll see how much power there is coming out of the exhaust.