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Basically when you're around people (not alone) and you really have to blast one, so much so that your stomache is starting to ache... so you let out a small amount of gas in hopes of it being a quiet or non-smelly fart. If it smells or is a blastus you hold it back, but if it's a quiet one you walk around and pass the gas in a discreet fashion.
Hung was in class earlier and he had to fart, he intended to let out a test fart in the back of the class but instead of is coming out as a quiet and/or non-smelly fart he accidently Sharted.
by JerryCurlPubes June 07, 2005
When you are in a social / public situation and have the urge to pass a considerable amount of gas. If it has been a long time since your last fart, you are unsure of:

1) The sound level (volume) of the fart
2) The smell level (stench) of the fart

... so you let out a small amount of the fart in a discreet fashion- thus you have let a test fart.
Frank was in church the other day and had to fart really bad. He said he intended to let a test fart but instead of a squibbler, it turned out to be a blastus and he wound up shitting his pants in the middle of the sermon. Damn, what an idiot!
by Frank Klaune April 14, 2005