dress lift
Lifting up a person's dress, usually with the intent to show off their underwear or bare crotch/butt. Usually this is done either by the wearer of the dress or someone nearby. It could also be caused by an inanimate object, such as a hook or other object that might catch the dress and lift it up. It can be done voluntarily or involuntarily (by another person/object).
If the dress is merely being held - not in a particularly revealing fashion - use the tag skirt hold instead.