The engine must be at top dead center where both intake and exhaust valves are closed (compression stroke). (mechanics.stackexchange.com)
Is it possible for the piston to be all the way up but you're still not at top dead center? (mechanics.stackexchange.com)
First, everyone who said that the braking effect comes from the compression stroke is wrong... the air in the cylinder is compressed which takes energy, yet after top dead center acts as a spring and helps force the piston back down, returning the exact same force as was put into it in the first place. (mechanics.stackexchange.com)