Sex, or sexual situations, where a girl is on top and/or clearly in control - no frowning, gaping, blank stares, etc. - usually a grin or lusty expression. Milder than femdom. Basically, she is screwing her partner(s) instead of the other way around. If the partner is also shown to be enjoying it, it might fit better in happy sex.
Inspired by comments on post #435995 requesting a tag for "sexually aggressive expression", although "aggressive" implies hostility or violence, which is not the idea here.