Once per turn: You can change this card to face-down Defense Position. If this card is flipped face-up: You can destroy monsters on the field, up to the number of other "Geargia" monsters you currently control.
Yugioh is a game where wording is extremely specific and extremely important. Select is not the same as Target, for example, despite both being basically the same action. And then there's Discard as Cost vs Discard as Effect.
In Geargianchor's case, it destroys multiple cards at once, so it doesn't target, even if you can select which ones to destroy in case you can only destroy 2 out of 3.
1) can you use its effect of changing it to face down defence position during ur opponents turn
2) if an opponents monster attacks it while it is face down and is forced to flip can you use its effect to destroy the attacking monster (preventing itself from being destroyed) or do both monsters get destroyed
(I’m a bit of a noob, forgive me if this is basic knowledge)