I learned that in Japan, the term they use for navel exposing is “chiller†which is a little strange, but it helps me find some content!
The post said “Belly chiller while stretchingâ€
Just another example of how an English word becomes a Japanese loan word while the meaning can completely change. If I had to guess, it's probably referring to how a girl would get a chill draft from their clothing riding up and exposing some skin. A quick search of チラ (pronounced "chira") shows mostly skirts getting blown up in the wind, so it's not specific to how shirts ride up.
For your searching pleasure, use the term 腹チラ (literally "belly chiller").