but how?
As a fireman I've attended many calls, one very similar to this https://video.nostr.build/cab30cdd54e2e4c93cda1e60405e1ea81d602b35e930224c153683803235c908.mp4
Discussion
As you sit down on the plastic chair your weight bends the plastic seat and makes the slot wider. Especially if it is an older or damaged chair. As you go to stand and your weight leaves the seat, the gap goes back to being smaller.
Ouch