Physical powershield storing (Previously known as rapid powershield, I believe)



Before you get your hopes up, I haven't found a way to store a projectile reflectbox, the following will only apply to powershielding normal, non-projectile attacks.





Terminology



GuardReflect: One version of Yoshi's action-state during pre-shield. GuardReflect starts when doing a full digital L/R press while able to shield immediately. GuardReflect lasts 6 frames and starts powershield window. It also makes Yoshi invincible and he can jump out of it, which enables parrying. GuardOn can be converted into GuardReflect if digital shield is input for the 2nd frame of GuardOn. Yoshi becomes vulnerable when doing so though.



GuardOn : The other possible action-state for pre-shield. Happens if shield is initiated with analog input (lightshield) or buffered digital shield input. Lasts 6 frames like GuardReflect. Yoshi can't jump out of GuardOn.



Powershield count : A number that's initialized to 4 when GuardReflect starts. The number normally ticks down by 1 near the end of every frame of GuardReflect until it's 0. If the Powershield count is greater than 0, powershieldable hitbox gets powershielded when it connects with shieldbubble. This can normally happen only during frames 3-4 of Yoshi's GuardReflect, since Yoshi's shieldbubble isn't up on frames 1-2. Note: this definition was invented to describe what seems to happen. I don't know how this actually works on code level.





Storing



Any time Yoshi's GuardReflect gets interrupted by anything while his powershield count is greater than 0, the powershield count gets locked up and stored to the value it was on at the time. Anything that intercepts the GuardReflect will work including spot-dodge, roll, jump, grab and shielddrop. For example, if you start a spot-dodge on the 2nd frame of GuardReflect, the powershield count gets locked at 3 because it was initialized to 4 at the beginning of GuardReflect, and ticked down once at the end of the first frame of GuardReflect. Likewise, if you spotdodge on the 3rd frame of GuardReflect, the powershield count is left at 2.



You can store up a powershield count of 4 by sliding off of an edge on the exact same frame you start GuardReflect. With other setups the storing is limited to 3, since it isn't possible to start GuardReflect while performing other actions. The wildest setup to store up powershield count of 3 I found is probably transitioning from GuardOn to GuardReflect, which makes Yoshi vulnerable and getting hit immediately. It's also possible to powershield a move on GuardReflect frame 3 and slide off of an edge immediately as a result of shield knockback (and/or shield [A]SDI). That way it's possible to store up powershield count of 1.





Powershielding with stored powershield count



When you have successfully stored a powershield count greater than 0, a number of next powershieldable hitboxes you block with shield get automatically powershielded. Usually the powershield count ticks down by 1 every time you shield a move, which means often you will powershield a number of moves equal to the powershield count you initially stored. Shielding a hitbox that cannot be (physical) powershielded and thus won't be, like a projectile, will still tick down the powershield count by 1. However shielding a move that causes 0 hitlag(puff's sing, <1% dmg hitboxes in NTSC 1.0) doesn't cause the powershield count to tick down.



Note that starting GuardReflect initializes your powershield count to 4 and it then starts ticking down normally, so if you perform GuardReflect while having a stored powershield count, the storage will be lost. Therefore after storing powershields, you have to get your shield up by lightshielding or by buffering digital shield input out of lag so that you start with GuardOn instead of GuardReflect.





Benefits



As many have noted here, physical powershielding unfortunately does very little for Yoshi as it doesn't allow him to cancel GuardOff into moves and neither does it increase shield knockback. As far as I know, the only benefit is not taking shield damage when hit, so one would perhaps be able to use light shield a bit more liberally when he has ps stored up, which can be done easily enough.





Stored ps depletion



Stored powershield count doesn't seem to wear off over time at all. The main ways of decreasing or losing the count is blocking attacks, losing stock or activating another GuardReflect. The most bizarre detail about the ps storing I discovered is that if Yoshi does a turn (Dash dancing counts too) or walks or performs side-b or successfully grabs an opponent (whiffing doesn't count), that seems to set the powershield count down to 1 if it was >= 1 beforehand. If you don't have a ps stored up, none of those options raises the powershield count though. There might be more actions that affect the ps count that I haven't found out yet, I'd be delighted if anyone can point them out.





Shield start-up deviation



If Yoshi has stored ps count, shielding a hitbox on frames 1-3 of GuardOn seems to use up extra powershield count: If Yoshi blocks on frame 1 he loses all 4 ps, on frame 2 the count decreases by 3 and on frame 3 it decreases by 2. I suspect this might be a counter measure to multi-powershielding with Yoshi, since on other characters the powershield count seems to tick down during shieldstun which Yoshi doesn't receive. However if Yoshi gets pushed off of an edge immediately after shielding an attack, the powershield count seems to always tick down only by 1 like normally.





Special thanks to Ponkapa for helping me confirm most of the details.