1. OTK is OK. If you dig into forums or websites about spanking, you'll run across the abbreviation “OTK,” which stands for Over The Knee, or in other words, “across the lap.” OTK is often the only “kink” an otherwise vanilla couple might engage in, hence the need to specify the type of spanking. (Other forms include, say, lying on one's stomach to be spanked by a standing partner.)
2. Spanking isn't always punishment. While some of us associate spanking with childhood discipline, as adults spanking can take on whatever meaning we like—or none at all. It can be fun to play ‘naughty student bent over the principal's knee’, but for some people it's just as much fun to order someone to spank them and spank them well. “You call that a spank? I'm not even counting that one. Try again. Harder, dammit!” Um...where was I? Oh. Right. Sorry. Spanking can also be simply about the physical enjoyment of the act, with no “dynamic” involved at all.
3. We're built for it. Let's talk about asses. Butts are essentially cushions, and as such can take a lot of force. It's hard to really seriously hurt someone during an open-hand spanking, especially if you avoid directly hitting the tailbone. At the same time, butts are sensitive areas that usually don't get played with very much, and just touching or stroking it can be powerfully arousing. Then there is the rhythm of spanking, which can often mimic a sexual one. When over the knee, striking the “sweet spot” where buttock meets thigh, it's easy for the spankee's hips to be ground against the spanker's upper thighs with every spank. Seriously, how can that not feel sexy?
4. It's hard to do it to yourself. In the quirky (and occasionally alarmingly inaccurate) film Secretary, Maggie Gyllenhaal's character attempts to elicit erotic sensations by spanking herself with a hairbrush. Her adorably frustrated failure illustrates another oddity of the human body: like tickling, spanking typically simply doesn't work as a solo activity.
5. A good spanking takes time. Spanking is an ideal way to start learning about erotic pain response. Starting gently, with a lot of time between spanks and other stimulation like rubbing or squeezing, prepares the body for accepting more intense play. It's what makes an erotic spanking different (and more fun) than getting whacked accidentally. Knowing when to increase the speed and hardness of play is easy: the rump will color nicely to let you know which areas are getting sensitive, and the spanker's hand will start feeling it when the spanks get intense. Follow this with a good slow comedown and plenty of time to rest and debrief afterward, and you're very likely to have a successful spanking session on your hands. So to speak.