I've been learning to Upsmash out of shield. I heard about Luigi being the best level 9 CPU and I was checking them out because I never vs. CPUs(and my testing easily showed that Luigi is by far the best CPU, computer's perfect reflexes+uptilted Fsmash/Firejumppunch = deadly combo. It short hops double aerial dair to nair and dair to tornado and everything) and the weegee ******* kept popping this Upsmash out of his shield and it was really pissing me off because it totally can screw over how much you expect to space on Luigi's shield and go unpunished.
Also, dtilt is Luigi's fail move, since I missed that discussion. It's kinda like "aw crap, I just accidentally used dtilt" sort of move.
Oh and to upsmash out of shield begin jumping from your shield and in the jumping frames input upsmash before you leave the ground. The hitbox(which is behind you) is out by the 8th frame, I think, which is fast as a rising Nair from shield.
EDIT: We're on to Dsmash already? Dang!
Dsmash is fast(comes out in 5 frames), does 16% damage, you don't need to save it to kill so you can spam it for it's damage, walk to Dsmash to slide while doing it is nice, on heavy/large characters it can hit twice at 0% for 32% damage, and for the same reason it hits twice you can also follow it up with an upsmash sometimes. The move is uncomfortable to DI for some reason, probably because of the weird trajectory it sends you at, and so it can be pretty annoying to get hit by for your opponent. The trajectory sends them up towards you and over your head(what Samus's Dsmash did in Melee actually). One of the weirder trajectories in the game. It's a really good move though because the ability to walk into a sliding Dsmash makes it much more comfortable to use and it has nice damage.
Oh and the fact that it has a hitbox behind is just a bonus, it's fast enough that you can just think about the front hitbox and ignore that it hits behind you too.