There are 2 methods to jump cancel.
The first method is to hold down 1 shift key, then release, and on the next frame hit the other shift key. Holding down the second shift will get you the height equal to a 1 frame jump, releasing it early can make you jump even lower.
The second method is to hold down 1 shift key, make sure NumLock is turned on, and then hold down 0 (or any other number) and release shift while holding down the number. This makes you do a 1 frame jump.
Hope this helps!