From the brochure:
It’s called the occipital drop (OD) because we use your occiput, a skull bone located at the back of your head. When your body is challenged (stressed) your occiput appears to drop a little on one side. KST practitioners look for or feel for the presence or absence of the OD when they put their hands on the back of your head.
 
-Comes in packs of 24, 50 and 100!