Reversals (like dps) work because they have invincible startup frames. You can take advantage of this to OS a block when you jump in on certain characters wake ups. You jump, do an attack, if they don’t do an invincible reversal, your jumping attack “touches” them (they either block it or get hit.) If they do an invincible reversal, your attack doesn’t touch them, because they’re invincible, and you just land. When you land, you have 3 frames (I think? I never get frame math for stuff like this right) where you can’t block. So if the opponent did an invincible reversal that had 4 frames of startup, you can block on that 4th frame after landing. Result: they don’t reversal, your jumpin touches them. They do reversal, you land and have enough time to block the reversal.
Timing for safejumping 4 frame reversals is strict. It requires proper spacing, and frame perfect timing. That’s why most useful safejumps use certain setups to make the timing and spacing easier (like the ultra 2, double dash, nj. hk safejump.) You can, of course, do the same safejump without the double dash, but you would be relying on yourself being able to get to the right spacing, and jump with frame perfect timing after waiting a long time. Shit’s really hard.
IMO the ultra two safe jump is still really hard, because it’s not just a matter of doing everything as fast as you can. You can either back dash > lp dp > ultra after the ex tatsu, or just dash forward > ultra, but either way, it requires a delay somewhere. The safejump on fei (forget who else) where you forward throw, whiff a stand jab, then jump forward mk is a lot easier, because you just press jab as early as you can after you recover from the throw, and then jump as early as you can after you recover from the jab.
You can’t safejump 3 frame reversals because of the landing recovery I mentioned above. If you jumped early enough for your attack to hit if they do nothing, then if they reversal you’re going to get tagged in recovery. Likewise, if you jump late enough to block after your attack if they reversal, then you’re attack won’t touch them if they don’t.
(EXCEPT FOR THAT SAFEJUMP THAT WORKED AGAINST 3 FRAME SUPERS WHICH IS STILL DRIVING ME CRAZY)
Hope that helps. Not really sure if you were referring to the safejumps haha, just kind of assumed.