"Since the increase of hormones and chemicals has produced a trance-like state, as play ends the submissive may feel out-of-body, detached from reality. As the sub's system stops producing morphine-like drugs, and as the parasympathetic nervous system kicks in again, the sub may feel a deep exhaustion, a sharp drop in temperature, as well as incoherence and un-coordination. In the lifestyle, this is commonly referred to as "drop" or "sub-drop"
Drop is experienced by athletes and adventurers. Drop happens to Tops and Doms, (though this is often less-well-recognized) for pretty much the same reasons as athletes and adventurers. It also happens to people after high stress situations. After an emergency like a car accident or a break-in, people often find that they go through days where they have a feeling of being adrift, rather than how they are used to feeling.
Drop can also happen if play is stopped abruptly. BDSM play is a very vulnerable experience for people. It often involves exposing one's inner-self in ways that one has never before done. Sometimes, inexperienced Tops will begin BDSM play, and then abruptly terminate a scene (perhaps because they rudely decide that someone else would be "more interesting" to play with) and walk away. This can leave the abandoned sub in a *very* down state -- feeling that they engaged their sense of trust to allow a Top to play with them, and that the Top simply let them splatter on the ground.
There is also a different sort of drop, which is a function of encountering contradictions between the ingrained (and often implicit) "rules" that people live their lives by, and the discovery that various things in BDSM make them extremely happy. Usually the last thing that people do upon discovering that they are ecstatically happy doing things which harm no one but which might run contrary to a moral code handed to them as a pre-cognitive child, is to haul out the moral code and examine if following it actually leads to happiness. "
See the entire article @ Chico Munch