Yeah so you're essentially holding your breath while you do it, so you'd need to take a break to breathe in and if you're trying to siphon you would need to seal your lips to prevent losing your progress / prevent the liquid from falling back down. But you can exhale while doing it, which also helps to make sure your don't accidentally inhale whatever fumes you're "pumping".

Granted it's kind of a silly thing to do and only marginally useful. Each cycle only moves a mouthful of air so I only find it useful for clearing trace amounts of isopropyl fumes from vapes and pipes without hurting my throat, but a fun trick nonetheless if you have the coordination. And depending on the fumes it might still burn your nostrils a bit.

Maybe the better way to explain the technique is: you can close the back of your throat and move your tongue back and forth to push air in and out of your mouth. You can also close your lips and do the same motion with your tongue to push air in and out your nose. All you really need to do is alternate between the two so that instead of pushing the air back out your mouth, you push it out your nose.