If the engine is revving, but the bike is not moving forward on the ground, then something is stripped or sheared off. When you lift the engine, drive shaft splines extend and engage enough to rotate the wheels. Also, there is less resistance so the wheels may turn easier, even if a transmission gear is broken. I've stripped these splines (the female side) My first suggestion is to pull the splined rear drive shaft out and inspect all sides. I answered a previous question on how to remove them if you search on my answers.