nostril flare

When the nostril's size expands.

Nostrils often flare when someone yawns or breathing heavily. It can also convey emotions such as fear, anger or disgust.

Some people can flare their nostrils whenever they want to, which some people might not be capable of doing. Some people's nostrils flare from talking or laughing.
Of course the nostrils can flare from sniffing or can be a prelude to sneezing most of the time like a twitch.

Her nostrils flared in anger when she saw what happened.

Stacey tickled Simon's nostrils with a feather, they flared before he sneezed.

"Dude, have you seen Jane's nostril flare?"
"What? No, why?"
"Fam, They're god damn huge."