Tap quickly to charge moves in PvP

I will usually speed-tap by alternating between my index and middle finger, or just tap with one finger using twitch muscles. To get the full charge (3 rings around the charge-up circle), you do need to tap at a very quick pace.

I don't remember exactly how much the damage modifier is from fully charging up a move, but I do know that if you don't tap at all, the move does 1 point of damage. Tapping only slightly results in meager damage. So it is important for getting the most damage from the move. The "tap as fast as you can" strategy doesn't change, regardless of what charge move you're using. If your opponent uses a shield, it will always prevent all but 1pt of the damage, regardless of how much you charged the move.

I tip with the very point of my index finger, without lifting my finger too much from the screen between each tap. (It barely leaves the screen surface!) Done rightly I always have three circles going and I can tap even more for 1+ second till the charge move goes off. I have seen the effect of my tapping on pvp my attacks do a lot of damage.

There's no "optimal" way because every individual is different, just find a way that works for you to reach 3 rings consistently. Personally I tap with three fingers, I know some do alternating fingers, some just tap really fast with one finger.

There's a significant difference in damage but I don't remember the exact numbers. All you need to know is that you have to reach 3 rings to deal 100% damage and there's no reason not to reach 3 rings.

There are no differences between charge moves regarding this, but it seems like it's slightly harder to tap after your opponent changes a pokemon.

It would impact shielding strategy if you for some reason had an opponent who doesn't tap for full damage, but I would assume everyone to always tap for full damage. Like I said, there's no reason not to.

I always do a single finger mash ala Mario Party. I find that when I do 2 finger taps it doesn't register as high powered but maybe I am doing it wrong.