A vibration.

I told my neuro, "It's like someone's taking a tight guitar string and laying it across my back, and when I bend my head down, they're 'twanging' it on my back, and I feel the vibration of the string."

That's as best as I can describe what it feels like in my case when I had it. It was in my lower back/hands to fingertips with vibration each time I bent my head down.

Wasn't painful, but scary at the time as it occurred before my MS diagnosis was even on the radar... however it was also the first symptom I had that really and truly pointed to MS.