It was kinda like a sneeze
- you know how your nose will start itching, and then ACHOO
! you sneeze
? Well, this was more like an allergy attack - my back kept having these sensations, which would build up, and then WHAM! it would arch, and then everything would start back up again. These days, it only happens every once in a while, and usually during a pseudo-relapse.
Personally I have no idea what it is. I do have myoclonus in my legs, and the sensations are similar, but not the same.