Before your comfortable passing wind in front of the person you are trying to impress, holding in all your farts means that the tiny fart molecules join together and form a fart poo. Smaller than a regular size poo, just a collection of farts forming one solid mass. Usually when the fart poo is passing through it is accompanied by a fresh batch of very loud farts which echo loudly around the toilet basin.
Tom: "I didn't want to fart in front of Naomi the other day so I held in all my farts"
Will: "Surely that can't be good for you"
Tom: "My stomach started hurting, and then I went to the toilet and deposited a fart poo"
"I've been holding in these farts all day now I really need to get rid of this fart poo"