Quite frequently now, the following occurs during my pranayama practice:
A continuous and vibrating electric like current will be established between the brow (and sometimes the hear chakra) and the root. This current can be quite powerfull at times and it stiffens my entire back. If I want to, I can spread my attention across the whole region between the brow and root and hold the current and feel the vibrations.
Sounds like classic kundalini symptoms that come about as the inner energies begin to stir. See lessons 54 and 69 in the "Main Lessons" link above for more details. Here for reference: