I'd tell you to look at the wiki but oh well... uh I guess I can't tell YOU that huh?
Shield happens when you hold D longer than X frames, Dippy or someone had an exact amount of how much circuit was drained while doing this and exact frame count. Shields do not count for a last arc and cannot be normal cancelled. If you do not cancel these, they have recovery time that's usually long enough for someone mashing 2a to continue mashing and hit you before you recover.
EX Shield is when you tap D. These count towards a last arc and can be normal cancelled. These recover faster I think and are pretty much what you want to do every time.
They both gain circuit on shielding, was 5% last time I checked.
EX shield scales 60% then 45% normally. Normal shield is 70% 45%.
EX shield when shielding a 5a low does 60% then 45% proration. That's normal, like normal ex shielding you don't get the scaled decrease until AFTER you hit a button. If you last arc like this you'll be okay.
Now, ex shielding a 2a high goes straight to 45%. You don't even have push a button for this to happen. The game instantly slaps down the proration, if you last arc like this your damage will suck. The game will set prorate to 100% and then it'll drop to 45%. Even normal shielding is subject to this penalty.