The Back Widow is a very tight transition from the Back Mount position going to the Spider Web. In the video above Eddie Bravo goes for the technique beginning at the 5:50 mark and finishes with an armbar.
At 10th Planet we drilled this technique a number of times. The way Eddie would do it is that we would work on 3 or 4 techs for a two week period. We worked on this tech in the mix a few different times.
This technique follows the principle of the clinch. Space is your enemy when doing this tech. You must keep everything as tight as possible.