Boomerang Physics Lesson
When a boomerang is correctly thrown, it is given spin at the moment it is released from the throwers hand. Spinning the boomerang causes air to rapidly flow over the airfoils creating lift that makes the boomerang fly. Because the boomerang is moving forward through the air, in addition to spinning, the arm rotating into the direction of flight creates more lift than the arm rotating away from the direction of flight. This causes asymmetrical lift or in other words more lift on one side of the spinning boomerang than the other.
Ok, still with me?
Any spinning object is subject to gyroscopic precession. Which simply means that any attempt to move a spinning object is translated at right angles to the original input.
For example - lets say your car is subject to gyroscopic procession (we will ignore the spinning requirement for the time being) and you drive it up on ramp to change the oil. As soon as the front end of the car lifts into the air, the whole car spins 90 degrees to the left and sits sideways across your driveway. Its either that or it flips over on its side, resting on the door forcing you to climb out the passenger window. I'm not sure exactly which way it would go but you get the idea, try and move it one way and gyroscopic precession makes it go another.
With a boomerang, gyroscopic precession makes it turn to the left, causing it to fly in a circle back to the thrower.
Or in layman's terms - magic makes it come back.