A fart the proceeds a larger fart. the reason for this is for safety of the individual farting, to ensure that there is not something more behind it. If the individual is not sure about if he or she has to crap or not, the sentinel fart is utilized to determine this. If the fart is disbanded in safety, it is ok to proceed with the larger, more robust flattace.
P1: Why does Brad keep crapping his pants like a 2nd grader? He is a grown man.
P2: He should probably be taught the magic of sentinel flattace.