I mean there has to be SOMETHING in there that is forming, and then being removed, right? I can't immediately crack my fingers again. There has to be a period of time, and it does appear the longer I wait, the larger the crack(within limits).
I agree that there must be something forming there. I can often 'feel' that a joint can be popped, prior to actually doing so, and sometimes, it seems, without even flexing the joint.
I can crack my fingers repeatedly by crossing my index/middle and ring/pinky fingers together respectively and closing my fist.