The ease of spinning a boomerang vertically around your hand compared to throwing it straight forward is due to the aerodynamic properties and design of the boomerang.
When you spin a boomerang vertically around your hand, it rotates about a central axis, creating lift as it moves through the air. The boomerang's airfoil shape and the asymmetrical design of its wings generate an upward force called lift, similar to how an airplane wing generates lift during flight. This lift force opposes the force of gravity, allowing the boomerang to maintain a circular path around your hand.
On the other hand, throwing a boomerang straight forward requires the boomerang to achieve both forward motion and lift simultaneously. Unlike a typical throwing object, such as a ball or a frisbee, the boomerang's design is intended to create lift and generate a returning flight path. When thrown straight forward, the boomerang's asymmetrical wings and airfoil shape can cause it to tilt or roll, leading to instability and a loss of lift. As a result, the boomerang may not maintain a stable flight and return to the thrower as intended.
The spinning motion around your hand provides the necessary lift for the boomerang, making it easier to control its flight and maintain stability.