Electric motors can spin in either direction. Most of our fuel engines cannot. The brushless motors can produce equal power either direction. Many fuel engines won't run at all "backwards". Few will run with equal power either direction.
You can start an engine by flipping it backwards. It might start and run that direction, but almost always will run poorly. When you flip backwards and they start and run well, they have usually bounced back from your flip, and are running in the direction they were designed to turn.
But what forcing the engine to turn on the other direction since i spin it in the other direction?
They have "backfired" from the wrong direction. You turned in the wrong direction, they backfired, and are now turning in the right direction.
Engines are timed. They fire at a certain time in their rotation. That firing is not at the center of the compression rotation. So it's "off center". And being off center, it helps the rotation in one direction, and resists it in the other.