I am playing this on Steam.
Not sure if anyone else has found this out yet but I tried it out a few times and I could get it to be semi-consistent.
For some reason if you hit one of the big jellies with an electric spear and then follow it up with a spearmaster needle to pull them out of the water they explode into hundreds of giblets. (See Image)
It doesn't always happen, but every time I've got it to is when I successfully hit the big jelly with the electric spear first and then pulled them out of the water with a spearmaster spear.
I tried it with the jelly mid shock, pulling the jelly out of the water while going into a tunnel (sandbox shelter), and by just pulling it out of the water normally with no other factors.
I first encountered it with mods but it is also repeatable in vanilla.