According to some brief research, it's a very short performance without a fixed format. Usually involves only a single joke or a humorous movement to amaze the audience or to cause burst of laugh.
For example: "I'm seventeen!" by you-know-who.
Old hag is oldDeserved to be called old hag
The performance is outdated
BBAHow's that~*Ippatsugei
Entrance fee: 100yenStanding on someone else is not easy at all!!Take it easy!!Grab it easyTake it easy!!Gap Ippatsugei
Ippatsugei: 一発芸 / いっぱつげい