The Behavior. So a guy's standing with his buddies on the street and a girl comes walking by. With his peripheral vision, he'll first subconsciously make a snap decision on whether she's attractive or not. If she is, he'll briefly stop whatever he's doing turn and look at her coming towards his position then go back to what he was doing. As she gets closer, his mind is timing her, and, when she walks directly in front of him, he'll stop and glance at her side. As she's walking away, he'll time it again and turn to look at her back side.
The Database. Every guy, through the course of his life, needs to evaluate every attractive girl he sees against every other attractive girl. His subconscious mind snaps a front, side, and rear image of the girl, all of which get thrown in long term memory. I think the goal is to average out the body types of every attractive girl he ever sees as his ideal body type, his ideal mate. It's also what let's him see the smallest body part of a woman and know if she's a potential mate.
I can see how guys who developed this ability increased their chances of finding a girl with good genes, meaning it's a trait that got selected in our population.
