A yukkuri inflating its body as an intimidation tactic (similar to puffer fish without the spines), often accompanied by them audibly shouting "PUFF!". Only works against other yukkuris, won't work against humans or any other animal. Even against other yukkuris it usually only works against koyukkuris. Against older yukkuris it often just angers them.
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 or even instantly kill opponents, all of which is obviously not true. In fact, often times puffing up actually puts the yukkuri in even more danger as if a yukkuri wants to puff with all it's might it usually needs to stand still and close it's eyes. This, along with their false belief that puffing up will get rid of the threat, will cause the yukkuri to be oblivious to an incoming attack from the threat.
While yukkuris will often flaunt puff-puff as their "strongest ability" most yukkuri seems to only use it as a last resort. If a yukkuri has another way to fight such as fangs of a stick they will use that instead. Even unarmed adult Marisas will often prefer a body tackle over puffing up
Humans sometimes also utilize puff-puff to scare a yukkuri, usually a koyukkuri. This can be done by puffing up their own cheeks, but is more often done with a baloon disguised as a friendly yukkuri which is then suddenly inflated when a koyukkuri approaches. Puffing up is so frightening to a koyukkuri that it can sometimes even traumatize them to death.