Here's a site with a picture of one.
http://www.rvpart.com/amazonstore/i...=1576&browse_id=90093&parent_id= You place the NM cable in grooves in the back of the device, just like you do for the ILSCO "insuleater" taps. Then slip the plastic retained over top of the cable until it snaps in place.
I was originally skeptical about these, but I've used them when adding a receptacle in my pop-up trailer and they are all over my manufactured home.
P.S.
They are listed by UL for use without a box, and the NEC addresses wiring of trailers and manufactured homes in 550, 551 and 552.
