It's not extremely hard guys lol, the average grab is like, 6-7 frames?
Now, there's no way I can spotdodge a jab/attack that comes out in 4 frames or less, and it's even harder if you can't see the animation changes clearly until the hitbox comes out. With most characters you can tell though and be able to react to the grab.
Oh well, I'll just let you guys think people can't SDI the jab or avoid the follow up grab or that people can't react fast lol.
I will now outline some things because apparently being skeptical is unfounded.
A grab will hit on frame 6.
A spotdodge is invulnerable on frame 2.
This gives you a 4 frame window to spotdodge a grab on reaction. 4 frames is 0.067 second.
The average human reaction time is 0.25 second, 0.20 second for athletes or gamers. I think I've seen one person who, knowing exactly what to expect, managed a reaction time of 0.07 second in a test.
But, let's give you the benefit of the doubt and say that you spotdodge only AFTER seeing the grab...
If you are somehow able, in that 0.067 second, to distinguish between a shieldgrab and pretty much any other action involving no longer shielding, you definitely are inhuman. I don't think this is debatable.
If you are not able to, then your reaction can be read and played. You're just as likely to spotdodge a grab on reaction as you are to spotdodge a roll, spotdodge, or simply a shield-drop.
Prediction is absolutely necessary to be able to spotdodge a grab once you see the grab startup, and you can't really say that the action he'll take is always a grab.
I remember a friend who told me he tried to punish those double shield into shieldgrab situations with Dolphin Slash. He also mentioned how it tended to get him punished when he reacted to a spotdodge or roll instead of a grab.
Now, this part is entirely related to using the Brawl hack of frame timing.
The first jab won't give you enough hitlag to SDI and escape from the second jab. As in, it's literally impossible even with frame-perfect play. It is, however, weak enough that it doesn't have the hitstun to combo into anything except second jab. The second jab can be SDI'd all sorts of distances.
So the next time you talk yourself up about avoiding grabs by SDIing the first jab, keep in mind you can actually avoid a jab-grab setup by normal DIing away... seeing as how there's not enough hitstun to actually keep you in place for the grab, ever.