Auto-cancelled FAir.
Basically you hit back to let go of the ledge, then at the exact same time DJ+FAir onto the stage. The FAir will autocancel. You have 8 frames to do the input, but the input itself has to be perfect. It's actually why my control scheme has Z-Attack, it was just a bonus that it makes DACUSing piss easy.
Easiest way to know if you're doing it right: The aerial autocancels, Derp.
Just note that GaW makes a beep SFX on frame 2 of his jump. If he makes that beep, you didn't DJ+FAir on the same frame.
I'm probably making it sound way more complex than it is.
Major things you could do with this go from the obvious (hitting them with FAir) to the shenanigansy (waiting a second, hitting with the sourspot, grab, initiate DThrow techchase).