So many compelling explanations....
I watched the vid again and would like to note some other factors, and bring in images to help explainify stuff.
My current theory:
Sonic hits shield, and then begins a turnaround(watch 0:36 - 0:38, sonic clearly turns around before hitting). Somehow, hitting the shield allowed sonic to roll off the stage for an instant, causing him to hit with ASC as the other Sonic falls off due to shield push. Though this shouldn't happen, because turnarounds usually don't allow sonic to leave the stage, it seems the shield hit might have altered something. As KID suggested to me, it might have been similar to that vid of the Sonic dong a Usmash and then falling offthestage after having it shielded.
Also, hm... You know how when the ASC has horizontal momentum, you can change its direction? I think that is perhaps what happened here. When the SDR became an ASC, infzy was holding backwards, so rather than moving forward, off the stage, it simply returned back onto the stage and instantly became an SDR for the second hit.
I would realllyyyy like to look into what can be done during phase changes.
Someone needs to replicate this in training mode at 1/4th speed.
EDIT:
During the shield hit stun, he travels off the stage but the animation for the turn around is not shown because it is taking place in the shield hit stun animation. His momentum can not begin to change while he is in shield hit stun, so he continues off the stage. This is where it turns into an ASC.
I like all of your theory about momentum, but as my screengrabs display, the turnaround animation
does happen
. At least the first half.
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So I did some frame analysis while I was writing this. I selected the most important frames IMO, although this is almost all of them anyways XD
1) Shield Impact Happens:
2) Umby's Sonic begins to fall off the platform. Also, notice the spin. This frame clearly indicates a turnaround. I compared it with a frame by frame of a 1/4th speed turnaround animation, and its the exact same shape:
3) The first hit begins. Notice how sonic is on the veryyy edge of the platform. However, In brawl, if you were that close to the edge, you would be off it. He is technically off the platform. Also notice the perfect roundness and slightly darker look of the ball, just like an ASC:
4) Second frame of the hit:
5) Third frame:
6) Fourth frame:
7) Now, this is the interesting part to me. This frame is immediately after the previous. there is no type of transition, it just
immediately. This is also when the second hit occurs: