If it's a (relatively) flexible German pen of a certain age, he may be taking advantage of the resiliency of the point to do stuff that isn't quite in the owner's manual, not unlike making the Kessel run in less than 12 parsecs or doing an outside loop in a Sopwith Camel; most people can't, few would think to try, but for a talented and lucky handful it is possible. That's definitely a fountain pen in the video.
I've made vast swipes with the back of a pen by rolling the paper over the edge of a table and moving the pen sideways, but I've never tried what's shown (wanting in both luck and talent, as well as forgiving old German pens). Richard Binder, before he left that line, did something called a Condor nib amendment, rolling the supra-feed part of the tines downward to create a useable broad back on a firm point, and the "fude" points for Japanese and Chinese calligraphy will produce a similar effect at a more conventional writing angle.
I'll also point out for those with vintage MBs that it stikes me as a GREAT way to remove any two-tone masking which may be on the point; try it under warning!