Think of the two arms of the boomerang as being just like the wings of an plane. The faster they move through the air, the more lift they generate. Unlike an plane a boomerang spins as it moves through the air and the combination of spin and forward speed means that some parts of the boomerang are moving faster than others. This means that the aerodynamic lift is not uniform over all parts of the boomerang. The wings of an plane are horizontal so the lift is upwards. The wings of a boomerang are sideways so the net lift is towards the center of the circle that you see the boomerang move on.
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