I can't reproduce this on MVS. I guess the timing is super-strict, because I can't seem to get it to happen.
Pun intended or not, SNKorSWM is gonna be pissed you beat him to the punch...
Also, it seems with this new double-jump discovery, people can no longer complain about the double-jump in the MD Shinobi titles
The uploader* says it was done with the CD version.
(*by which I mean AM)
Translated instructions from NeoGeoKult:
Double Jump: Press jump and shoot with a very slight shift (jump slightly before firing), then again when the first shot is fired. The timing window is very tight! Note: This trick only works with Elta, Shinobi and Waterman (easier to do with it).