On the beach, when (normally) a girl (usually hot), wants to seduce or just pleasure a guy on the beach laying on a towel, she walks up to him and says "your towel is twisted, lemme fix that." She then proceeds to straddle his head and squat down so his head is directly under her ass and she pulls on the towel near the swim trunks area to slightly pull down the swim trunks, providing a double sensation.
If the girl is REALLY hot, or the guy is seen getting a boner/liking it, the girl can touch her ass to his face.
This is normally done with the girl in a bathing suit, often a bikini.
Dude, this insanely hot girl in a bikini gave me a beach ass-view today! She sat on my face!