Barbarella want to test, how long her slave can be without airsupply.
She steps over him, wiggles her ass above his face and let him take some fast breath-sips for start, the she decends and starts the stopwatch and begins to count.... 10 seconds.... 20....
Without air, the seconds are very long, and he needs to struggle until Barbarella tinks he must breath and go up for some air....
Soon she decends again her ass and seals his oxygen supply properly, the test can start again and with prolonged "without air time" she thinks she can improve his capability.
She's an effectively coach in breath-holding.