Turning a prop backwards causes the oil to reverse direction and air to enter.
Depending on where it ingests the air is what determines the severity of the problem that may arise at next start.
I parked a car on a hill and put the manual transmission in a forward gear.
When I came back out, it had moved down the slope about 15 feet.
That was enough to slowly draw the oil out of all the hydraulic lifters and tensioners and the engine almost threw the chain off and the lifters clicked for about 10 minutes afterward.