shirt pull
When the collar of a shirt has been pulled down, usually to expose the breasts.
Not to be confused with shirt lift (lifting up the bottom of a shirt to expose the chest) or shirt tug (tugging down the bottom of a shirt to cover the crotch).
Also, not to be confused with collar tug which is for when a character pulls out on the collar of their shirt merely to let fresh air in or to show nervousness, although if it results in the exposure of the breasts or cleavage, it should be tagged as both.