It's pretty easy to turn the wheels because of the differential. You're turning one wheel, and the other rear wheel will rotate the opposite direction. In other words, you're not rotating the engine -- just the internal gearbox parts. If you want to turn the engine, you need to lock one of the wheels (typically I use a breaker bar and 19 mm socket, and since my car is on a mid-rise lift I use the floor jack handle wedged against the floor). It's easiest to move the engine when car is in a higher gear, like 5th. At that point, it's still pretty easy to rotate the one wheel and turn the engine.
Edit: beat me to it