The spine is the central nervous system HQ
..and is one of the places one "feels" the presence of external entitites.
Just even being aware that this is not a "normal" physical sensation, and that this is an external influence, tends to short circuit it and put you on your guard.
It's very similar to the raising of the hair on the back of the neck when one speaks of the dead. The central cortex is a transmitting receiving device and any nearby entities suddenly perceive you on their "radar", when most of the time they don't really see.
Their perception, you see, is signal based not visual based. This is the same thing that draws them to one as one sleeps - dreams are closer to their present reality and as you sleep...you are broadcasting.