I think it's just that rugose surface on the outside of the inner tube of the tip. The longitudinal ribs that you can see the end of in that picture. They go the length of the tube.
I find the Final-E tips grip really well, but as usual my ears fall between the M and S sizes.
The small Final-E tips fit me perfectly when flipped, super comfy, deep fit, wide opening, great seal, but those ribs make them unable to grip reliably to any iem I've tried them on (whilst inside-out at least) and I've gone as far as to try o-rings and even tying them on with dental floss and thread.
After a ton of experimenting I've settled on the stock small tips that came with my IMR EDP's, flipped. They stay on MUCH better.
That's what I'm currently using on both my EDP and Shuoer Tape, but I only have one pair, and for whatever reason they aren't quite as comfy as the Finals, so I would still like to find something better.
Still hoping someone in the UK will start stocking Xelastecs in single pairs. Everyone abroad either seems to charge a fortune for shipping (on top of an already high price), or have a kanji site which I can't navigate.
I wish someone like Amp-3 would get them in.