A yukkuri inflating its body as an intimidation tactic (similar to puffer fish without the spines). Only works against other yukkuris, won't work against humans or any other animal. Even against other yukkuris it usually only works against koyukkuris.
Yukkuris will puff at anything that doesn't let them take it easy, even inanimate objects (e.g. if it's raining the yukkuri may puff at clouds).
Some yukkuri delusionally believes that puff-puff can actually deal physical damage which is obviously not true.