Tap to throw at the spinning log. Land them all without hitting one that is already stuck.
Tap anywhere to throw Never hit one already stuck The log changes speed and direction
Keyword
The rim is the whole game
A log stands end on and spins. Tap anywhere and your throw goes straight up into whatever part of the rim is passing the launcher, so the only thing that decides where it lands is when you tap.
The log will not hold a speed. It speeds up, slows down, hangs still for a moment and turns back the other way, and it does all of that on a pattern that is different every time.
Land every throw the round asks for. Hit one that is already stuck and it bounces off with a clank and the round is over, so aim for the widest piece of clear rim rather than the nearest one.
A bounced round costs you nothing but the round: you get a brand new log and as many tries as you like. Clear every round and the cache unlocks.