A TENs unit can be a powerful tool in your coping toolbox. Applied to the back and controlled by the laboring person, it adds a physical distraction as well as a mental action.
Apply one pair of pads at the mid-back - align the top of the pads with the bra.
Apply the second pair to the low back - align the bottom of the pads with the top of the bum-crack.
Both pairs should sit about a pad's-width away from the spine.
Turn it on slowly and find the right level for you. You can leave it running at a low level between contractions or turn it all the way off.