Elaborating on the above tech, I've found some stuff that makes it even more deadly... it's basically the same set-up, after mb-command throw you instant air dash forward then do j1, and depending on the timing you hit in the front or the back. These are my notes on the cross-up j1:
-There are 3 primary timings on the j1:
1) early timing: you hit in the front, on hit you land behind your opponent, on block you land in front
2) later timing: you hit in the back, you land behind on block or hit
3) small window between early/later: you hit in the front, you land behind on block or hit --- this is the fun gimmicky one, I haven't tested this one on all hit boxes yet
-You can hit confirm a standing 3: (this is the most exciting option)
j1, 3 can be visually hit confirmed into slide (or mb iceberg versus grundy, bane, lex or doomsday). This means you can attempt a 50/50 cross up that, if blocked, can be confirmed into a blocked 3, which "guarantees" another 50/50 into f3 or slide.
-You can also hit/block-confirm with f113, 111, etc.:
similar to the above, you can j1, f113 which is easy to confirm into a damaging combo, or if blocked you can just as easily confirm into a d1~slide frame trap, a throw, a mb.f3 or whatever you want with your +4 frames
-You can cancel the j1 into slide for a high, low or (if you have the meter) you can cancel into bc.f3 for a high, high --- obviously this option is the most costly and most likely least useful