Ok, let's use the KISS method
1. The RPM will drop noticeably
2. every other engine parameter will drop with the RPM
Why? no fuel=no combustion=no turn
The propeller will now be driving the engine which will be acting like a brake due to compression in the cylinders.
The RPM will vary depending on airspeed and blade angle and engine type.
What won't vary is that you'll be trying to feather the prop/restart engine and hold the a/c level(twin), might not be as noticeable on a fourbanger but you'll know pretty darn quick.
Hope this helps.
FM