Well look at it this way.
Anyone who has ever done shrooms know that they grow under cow shit. The cowshit and the ground provides a perfect place for them to grow. When the shrooms grow, the poo particles of the cow shit get on the shrooms, and the shrooms use it as a defensive weapon by turning it into a poison. When animals go to eat it they get affected by this 'poison' (and get high). After the animal has experienced that poison they know not to eat it again.
Humans use that 'poison' on the shrooms to feel their effects.
Jet is basically the same shit, only your directly inhaling the brahmin feces particle fumes, which would most definitely provide a much stronger effect on the user than that of shrooms.
The people who use Jet inhale the fumes of the feces particles (mixed with some other crap like fertilizer or something), and they get high. The feces particles then works the same way on the brain as eating shrooms would.