One of the sensitive spots of the domestic cat’s anatomy is the area of the vertebrae just before the base of the tail and at the end of the back (commonly described as the “base of the tail”). Cats respond to being petted slightly stronger than normal at this point of their body.
The domestic cat tends to get a bit more excited when this particular spot is being stroked with a bit of pressure. In the video below, the cat certainly does get excited as can be seen by the gurgling noise she makes and, slightly strangely, she obsessively grooms his right foreleg and the hand of the woman petting him. Note: over time the video may fail to work. This is normal. Sorry if this has happened.
We know that cats will be motivated to lick themselves and their human caretaker when the owner strokes them. This is probably simply because stroking is like licking and they are returning the favour and in the meantime they are motivated to lick themselves as well.
In my opinion, the strange behaviour in the video is simply an extreme version of what is normal (self-grooming with more vigour). As for the noise that she makes, it sounds as if it’s an expression of excitement simply due to the fact that the nervous system has been excited by the firm petting at the base of the tail. What do you think? I am sure that you have experienced something similar with your cat. Sarah Hartwell guides us as to why this spot has a significance in her comment.
At the end of the vertebrae of the back just before the base of the tail there is a flat spot. I’m sure that you have noticed. That is the sensitive spot which makes a cat, sometimes, do things which appear to be slightly strange but which are normal but done is a more excitable way.