Illusion flip is what you call one of them late 90's hardlips that is more like a weird pop shovit between the legs.
It's half pop shoveit, and half hardflip and it's hard to tell if it's really even flipping or not.
The term illusion flip dates back to the Chad Muska/Donny Barely days.
These days it's like a sideways diss to call someones hardflip a "illusion flip", but I remember getting laughed at in the late 90's for still doing my hardflips the right way when everybody else was doing illusion flips.