Tractors pull, pushers push.

When an engine is on a model in the usual fashion, pointing forward, the prop rotates counter clockwise. And the tractor props we know and love, and are often the only props you can find in mose LHSs, create pull. If you look at the airplane from in front, where the prop is going to pull the airplane, you see that the prop is going to rotate counter clockwise.

Now point that engine aft and stick it somewhere on that airplane to work as a pusher. But stand in front of the airplane where the prop is going to now push the airplane. And which way will you see that engine turn, and that prop spin? Clockwise.

So it's obvious that you've got to do something to the now aft facing engine/prop. Because if you run it without change, that engine/prop are going to pull the airplane the wrong way.

So what if you just turn that prop over on that aft facing engine and bolt it down "backwards"? Nothing much, because the prop blade still bites the wrong way. Now however, it'll bite poorly because the airfoil is backwards.

So you need a prop that's made to work right with engines that are pointing the wrong way, so that you don't have to do anything to those engines and can use them simply by putting a pusher prop on them.