When your body is attacked or in pain the brain releases chemicals, called endorphins, which act as natural painkillers. Once the body is used to the pain and the "attach" - my hand - continues, the endorphins build up in your system. The feeling is often described as floating, and can reach a point where the spankee is "spaced out" or in "sub space".
It's a high as powerful as any drug - not that I've tried the serious ones. Even I get it as a Spanker. Sometimes at the end of a spanking I'm a little bit spaced, so it's best to wait half an hour before accepting a lift home with me.
The downside is that sometimes, when the endorphins wear off, a sudden mood swing occurs, causing enui and a feeling of loss. This is known as "sub drop". Nasty.
The best way to get to sub space is a slow, gentle spanking, building in intensity, maintaining rhythm and closeness between the parties. Your needs may be different.
Seriously I have used this type of spanking, also known as a "good girl" spanking (not that there's any such thing) to help a friend de-stress before going into hospital for major surgery. There's a whole other blog entry to be written about going to A&E and realising you have fairly fresh can stripes! (Don't worry, they've seen it all before).