Most likely a counter-hit. It may also be possible depending on the character, just with a smaller window for the link to work.
Pretty sure invincibility time activation is the same universally. However, some non-charge characters are at an advantage, since they can start doing special move motions when the screen is still frozen. So for example, Sodom gets be invincicible while moving almost all the way across the screen (FP Jigoku Scrape), and most other characters can’t. So even if all characters are invincible for the same amount of time, some characters get more use out of it.
Charge characters can’t do that, but they can cancel a regular move into a special. The Vega player(s) in the a-cho vid appear to be doing the flipkick immediately after activation for anti-air, but if you pause the vid you can see that he actually cancels a crouching Short ASAP. Flipkick has invincibilty at the start anyway, so even if it were a slower move, it would be pretty safe.
Like with the Sodom example, you have to take speed of movement into acount, and also distance traveled. Like, wallking through fireballs- if you attack instead, you may get hit, even if your attack reaches. a) time stops a little when you hit someone, and b), the attack (fireball in this case) is still there, and your invincibilty has run out.
On the other side, if someone does an attack, but the move doesn’t actually begin until after the screen unpauses, or continues for a long time anyway (like 'Gief’s splash or Gen’s hand slap move), your invincibility can run out. Likewise, there are some moves which hit later/for longer than they appear… Let’s say Shoto sweep is 5(5)9… hits on it’s 6th frame and can hit up to the 10th, but after that has 9 frames of recovery.
So you walk forward and activate V-ISM (with 50% meter) on it’s 1st frame. You’re invulnerable for 12 frames after that, so you can do whatever- let’s say the BAS OTG, which starts with Akuma’s sweep, which also has 5 frames of startup. Basically, you’re safe up until the 12th frame, and he can only hit you up until the 10th…because you activated V-ISM a frame later than he started the sweep, you have 3 frames to spare instead of one.
But Guy’s sweep is 7(19)18. You can still activate-sweep, but if you activate at the same time as Guy sweeps, that only leaves you with a two-frame window to actually hit- and that’s assuming you did the sweep as soon as possible, hitting the button on the first possible moment it would register. The attack isn’t over, is the point. So if you’re off by a milisecond or have to walk forward or something, Guy’s attack is still active, and you can be hit or trade. Even if you activate one frame late like in the example vs shoto sweep, that only gives you a 3-frame window which is really small- if press jab with most characters, the time between when the attack starts and when they actually get hit is about 3 frames.
Yeah, anyway, short version: sometimes you have to activate late. Using quick attacks helps, like crouching Short xx sweep into the OTG.
Invincibilty time after activation does vary depending on how much meter you have, though. Starting from 12 frames if you activate with 50% meter, up to 18 frames if you’re at 100%.(**)
For examples of time in frames- there are roughly 60 frames in a second, but since the game is played with Turbo 2 as the default setting, it gets harder to tell, I think.
-Sodom’s sweep is 43 frames
-whiff Jab DP with Ryu is 48 frames
-when you activate V-ISM, the screen goes dark for 30 frames
-Sakura’s whiffed b+FP is 20 frames
-Ryu’s hopkick doesn’t hit until the 20th frame
-hitstun/blockstun from a Jab is about 13 frames
-when you throw a fireball with shotos, it takes about 12 frames before it can hit. A regular fireball with Akuma is 12 frames before it hits, the red one, which seems like it takes forever, is 18.
-Most standing/crouching Jabs hit on the 4th or 5th frame
**(you have 144 “dots” in your super gauge, altogether- To find out how many frames of invincibilty you get, the equation is that number divided by 16 plus 8 (n / 16 + 8). So for example if you activate at 50% (72 dots out of 144), you get 12.75 frames, but rounded down to 12. At 100% you should only get 17 frames (144 / 16 = 9, +8 is 17), but you get an extra frame thrown in for being at 100%, I guess.)