I watched that video, but it's difficult to see what's happening. The chatter made by the stone makes me think that the high points between serrations are being worn down and the full edges of the serrations are not being reached. After a while, I'd guess that the cutting power of the serrations would be lessened.
This method is nearly exactly how Spyderco tells you to use the SharpMaker to sharpen serrated blades. Also essentially the same as using the 701 (M or F) sharpeners, which are shaped very similarly to the stone in the video.
At one time I carried a serrated Delica and I used it quite a lot and sharpened it like a sharpening obsessed person that would make long posts on a knife sharpening forum.
. It was nearly always phonebook paper slicing sharp. After a while I noticed that it didn't have the "bite" like it first did when doing a pressure cut. When I first got the Delica, the points were extremely sharp and cut into things like plastic blister packs most impressively. After a year or so of sharpening it all the time, the points were rounding off. Not totally dull and round, but no longer impressively pointy either. The scallops would still slice phonebook paper.
I gradually figured out that I had "ruined" the profile of these serrations with this method of mostly only sharpening the scallops and using the "chatter method", which encourages rounded off points. Later after reading posts from Jason B, I realized that there were many ways to approach this, including Jason's way which is roughly:
1. Grind the flat back side (not the scallops) at a very shallow angle (5 to 8 degrees as an estimate; nearly flat) until you detect a burr being formed in all of the scallops.
2. Remove the burr on the scalloped side using a sharpmaker, Spyderco 701, tapered rod, or something else appropriate.
3. Maybe a few more strokes on the flat side if #2 has flipped the burr to the flat side.
This seems to keep the points sharper for longer. I've restored at least one rather abused serrated knife this way and it worked well.
Individual grinding of the scallops is probably even better at keeping the points pointy, but requires a ton of attention to detail and probably a jig or fixture (or maybe just a clamp and a tabletop) in order to do it well.
Brian.