letting the air out of the tires

(v). To fart subtly, and slowly. Usually it is a fart that is restrained to the point where all you can here is a wisp or a faint hissing noise for a few seconds. This fart is not a ripper, or a fanny flapper. It's a breezy one, that can take on the harshest stench depending on what one ate beforehand.

When you think you have to take a shit, only to sit on the toilet and just fart, without actually shitting.