I find myself using this move more and more considering the potential it has for mix-ups (especially if you have been using 4B for constant tech traps. Your opponent will most likely tend to stay on the ground for a second or roll. You can the use this move to pick them up, although it works on some more of the time then others (always on zas). But if you can get the first two inputs in, I find it rather easy to get the JF, but that could be just me. One method I use when I play to make this move a bit more consistant, is set one of your bumpers or triggers, to something with B in it (ex: my right bumper is set to A+B. It seems I can press that alot more accurately than on of the buttons on the controller. Of course disregard the last two sentances if you play on stick. There is no easy trick to getting this JF, but if you practice it and use it alot you will get the rythem to the move and use is much more than 20% of the time while playing.
Edit: try and watch for the the second vertical hit right when it reaches a high enough point to where it connects with your opponent. If none of the above works just go into training and practice it a couple hundred times, eventually you'll get into such a good rythem that you'll do the JF without have to connect with the other two inputs (this still does ~45 dmg so it's still a useful move, just unsafe.