As the boomerang flies through the air, each wing produces lift. Once again, Bernoulli's principle is used to explain how the lift is formed. The air moves faster over the upper surface than the air moving over the lower surface. This means that a pressure differential exists between the lower and upper surface which translates into lift.
A boomerang is thrown with a spin in a similar manner as the discus and frisbee. This spin has two effects on the boomerang as it travels through the air. The first being a stabilizing force known as gyroscopic stability.The second effect of the spin results in the curved flight of the boomerang.
The turning force imposed on the boomerang comes from the unequal air speed of the spinning wings. If we start with a stationary, spinning boomerang, both wings would produce the same amount of lift. Now give that same spinning boomerang a forward velocity and the speed of the air traveling over the wings differs. Thus, the forward moving wing experiences more lift than the retreating wing. The net result is a force which turns the boomerang. Due to a phenomenon known as gyroscopic precession, this force is felt 90 degrees from where it was applied.
This gyroscope is mounted so that the axis of rotation can move freely. When the gyroscope is spinning, it can be moved and turned without changing the orientation of the axis of rotation.
A force on one of the mounting rings will change the axis.
A weight hung from a mounting ring will make it precess.