It could have something to do with the fact that the nerves in this portion of the spine are connected to the pelvis or the vulnerability factor of being touched from behind that makes this area so sensitive.
Whatever it is, the slightest touch here can evoke pleasure. Tickle the area with a feather or your lips and tongue.
Feeling adventurous? Try an ice cube, a vibrator, or pinwheel for some sensory play.
