Last week Master gave me the first spanking of 2013. He seemed, from my perspective (which at the time was bent over and face down into the mattress), to focus on the "Sweet Spot."
Again, from my perspective, the "Sweet Spot" is the area where your thighs and bottom meet. It's the part of your butt you sit on, when you are sitting upright with proper posture. It's the part of my ass that least enjoys being spanked (but that's beside the point).
The point is, to be specific, hitting the "Sweet Spot" serves a dual purpose. It's multitasking, if you will, which is efficient. Master loves things that are efficient. So, when He first hits the "Sweet Spot" over and over and over, until it hurts/stings/burns/is-on-fire (take your pick), He gets pleasure (He is a Sadist, after all).
But then, when He takes a break from the spanking so He can fuck me from behind, He causes more pain to the "Sweet Spot." When He enters me , His pelvic region meets up with/presses against/grinds unmercifully against the "Sweet Spot." This in turn causes more pain/moaning/whimpering from the slave girl, all of which provide Him with even more pleasure.
Efficiency...it's win-win for everyone. Master gets to double His pleasure and double my pain.