yea man you do not have a long time to do this. The time/frame window is kind of short ( depending on where you are standing)
IF you watch very carefully, ( when being crossed up ) you will notice there is kind of a half way point when some one crosses you up. Once they pass this invisible vertical line, your character will change the direction they are looking, to that of where the opponent will land. IF you are dashing when they leave the ground, and keep dashing before they reach this half way point, then you will do the proper dash and not the one you are getting.
- If you are right beside the person (like you're walking and its pushing the other character), then you have a shorter time/frame window to dash under them.
- If you are a little spaced so that when the opponent finishes their jump it barely becomes a cross up, you have a lot more time/frames to dash under them.
The latter one that i am talking about, is much easier to land. this is because that half way point i am talking about, is above your head and if the opponent is further from you, you half more time to complete actions before your character changes the direction they look towards.
Edit:
Qwark28@Metzos
if you read what i just posted, its not far off from that super dash Jer and I were doing.