It happens if you hit [Up], [Jump], or [Attack] while charging side-B or down-B on the ground.
we call it a vertical spin dash jump (VSDJ). It has the same exact properties as a normal SDJ (jumping from the ground rolling part of the spindash), except it starts off going straight up.
This maps out all the possible 'phases' that the move goes through during and after you release the charge. If you have a double jump, you can jump to cancel any of the aerial phases. Hitting
[Up],
[Jump], or
[Attack] out of the grounded phases (charge, SDR) makes you go into the spindash jump (SDJ).
Click the blue arrow thing in the quote for more detail.
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Just a note for any future 'pioneers':
If you think you've found a technique, post it in the AT's thread that TwinkleToes has stickied, or if it's related to spindash, post it in the side-B/down-B thread.
It keeps you from becoming a public example of what not to do, especially if some cranky member who keeps waking up to fire alarms late at night decides to flame you for trying.
Also, chances are that we've already found it and it's under some other input/name or something.