your instinct makes sense--what you are describing was actually our first implementation of the mechanic. the insight that you might be missing is is that it's really the accumulated speed of the bead that determines the spin. think of it like a golf club or a tennis racket maybe--first you push the mouse up to 'send it' up, then you pull the mouse down to 'whip it' down, releasing LMB at some point during that 'swing' to fire.
to get the maximum spin possible, you have to 'swing through' the center of the screen and release LMB right in the middle--that's when the bead is moving the fastest. do you think a different gif on the page or a video or something would help?
the spin is indeed vertical-only, although this is relative to how leaned you are--i.e. if you lean all the way to the side, you can spin 'horizontally', if that makes sense.
really appreciate the feedback. thank you for your time!