When you reverse a slow speed engine, first the engine has to stop, and that should include all rotation transmited back from the prop still running through the water.
If you are trying to do this in automatic the system will most likely not allow a start until the engine is completly stopped.
If you are 'On the sticks' in manual you may be able to break or stop the engine on air.
On older engine the whole camshaft had to shift forward or aft, to change the timing of the fuel pumps and the exhaust valve. (and of course the air start distributer)
Then it was just the air and fuel cam.
Now with common rail, its probably just the air start distributer, (I havent had the pleasure yet)
Hope that helps