Not sure, but I do vaguely remember one of my chinese friends explaining a kids custom where they'd put their fingers together like that and the other child was supposed to break it, signifying that their friendship was over.
okay, let me put it another way. Ritsu is trying to get her fingers really really close together without touching, and she was annoying mio with it. So, mio had enough and forced them together.
W-what do you want? How boring!Tsk...Raaawrrr~hmpftouch