Reddit has answered this for me: Also other websites. I also managed to do it myself. 'It's some kind of gestural tick that psyches them for battle.

Kaneki started doing as a result of Yamori always doing it when he was torturing him, and Yamori picked it up from the human investigator who tortured him. Naki also does it, on a different finger as a tribute and to mimic Yamori.' Source: Reddit user: “ Cracks Finger” is a gestural Tic shown by Ken Kaneki, Naki, and Yamori. It's a habit started by Yamori's torturer, then imitated by Yamori, Naki, and Kaneki.

Sep 24, 2014 - Well when your fingers crack that means that you're releasing air from in your joints and excreting it from your pores. Usually when I'm in the.

In the anime “Tokyo Ghoul”, Yamori cracks his finger every time he tortures Ken Kaneki. This teaches him to crack his own fingers after his escape.

I didn't want to do this but here (Text below reddit post): Image found here: An actual video of someone doing it.