You know the scene. A dopey guy is in the same room with an improbably hot female who wants to jump his bones. Instead of leaning in for a kiss or going forward with a full-on embrace, the woman finds herself stripping off all of her clothes in one smooth motion. As the dress drops to the floor, all we see is a pair of comely knees and the slack-jawed expression of the guy who was up to this point completely clueless of this woman's intentions. Without any full-blown nudity, we're left to wonder how magnificent that unseen naked body must be to earn such a dumbfounded expression on the hapless male.
It works even in certain R-rated movies that have no explicit sex scenes. In The Big Lebowski, Julianne Moore shows up at The Dude's abode unannounced, ready for the physical act of love. After she disrobes, the scene jumps forward past their zesty round of coitus and we see them relaxing in bed together, and it's left to us to fill in the gaps. Most Sexy Clothes-Drops are the same, though sometimes we're given a little bit more of a visual aid.
In this Buffy the Vampire Slayer episode, we are treated to Xander's speechless bewilderment at Anya's sudden nudeness. In case you didn't understand what was going to happen after this, that spooging juice box should give you a hint. Again, it's TV so we don't see any naughty parts, but we're meant to assume that you'd Ocean Spray yourself too if put in his shoes.
It's such a bizarre and unrealistic ritual that almost no one has experienced, yet we accept it so the story can move on with the implied ugly-bumpin'. In this particular Buffy episode, those bangin' ankles belong to a 1,000 year-old former demon without any idea how human sexuality works; that's this is the most reasonable explanation thus far says a lot about the Sexy Clothes-Drop.
Avoiding nudity is key to coming in under that R rating. According to the MPAA, nipples are off-limits if you want to come in under that R rating, but everything else above the waist is fair game. The first Tomb Raider is PG-13, for instance, but it's got enough sideboob to start a war.
You can show every angle of a woman's chest all day every day, but if an areola slips in there, it's game over. Children of America will writhe in agony the very moment they see a slightly discolored snag of flesh, despite the likelihood of them owning a pair of nipples and/or at one point used their mother's mammaries as a primary source of nourishment.
Going PG-13 means you gotta skirt that line -- you need to maximize the titilation without making people feel like they should be watching in their computer chair with the sound turned down. This happy medium of half-nudity has become so common that it's often found in R-rated movies like There's Something About Mary.
Whether it was the directors' decision to keep Mary's innocence intact or there was a non-nudity clause in Cameron Diaz's contract, frontal boobage was not an option for TSAM. So they went for the next best thing, which is getting one atom away from a nipple and then moving back.
Also see: The Strategically-Placed Arm
In the Nicolas Sparks cryfest The Best of Me, there's a pivotal moment where a young couple becomes intimate for the first time. It would be unconscionable for movie sex to look enjoyable, so the dude's arm is carefully held in an area that prevents people from being traumatized by the sight of the natural human body.
Someone gets shot and dies in The Best of Me. You see the bullet hole in the forehead of his corpse. But thank goodness for that savvy arm placement, or this movie would've been inappropriate for minors.