The GIF is sped up. I actually suspect it has a chance to work, but not necessarily all the time.
When it was put on, the water was not literally "to the brim", though it was close. Still enough that if applied, as more water fills the bowl, the water would push the air into the film and create that bubble. Pressing that, and letting go, should create a limited-strength positive and negative pressure sequence. The drawback is it should be weaker, but the benefit is that it is total and complete - due to bowl design, you may not get a solid complete lock with a plunger, or not oriented in the correct direction. Since the entire bowl is used, direction does not matter since there is literally only one direction for water and air to go - a plunger could lose much of its force into the bowl instead of down into the trap.
I think the GIF just skips quickly to the result - you should be able to clearly see, and feel, if there is still water at the almost-overflowing level.