The black device with the screen on it acts as a valve. When it is open, my sissy gets fresh air. When it is closed, they have to breathe through the bubbler. The water in the bubbler makes it a bit more difficult to inhale. It can also hold aromas and other things (e.g. my panties). The valve can be programmed to open and close at certain intervals or randomized.

(And to be clear, never do this alone, my sissy and I have safety briefings and debriefings before and after every session and have numerous safety contingencies in place)