This generally happens when someone has been working out or stressing their torso and then a little poot comes out with an amazingly awful stench.
Holy crap, Lucas was exercizing and then he smelled horrible i think it was an air fart.
Farts the are made by trapping air in your anus. Accomplish this by relaxing your anus muscles, usaully in a bent over position, thus allowing AIR to enter your butt. Then closing your anus, and letting it rip whenever you desire to.
If you wanna fart but dont have the gases inside you to do so, just practice airfarts