Washboarding

When I came home from work this evening, I went to look at the bees. There was a cluster on the hive porch acting in a way I had never seen. I ran into the house, got the camera and took this video. As you can see, the bees are rubbing the surface with their forelegs as well as licking or biting with their mandibles. I watched them for some time, wondering what they were doing.
I had read about a scent gland, the Nasonov gland. At first I thought they might be spreading there pheromones. So I got close and smelled, but only smelled the usual hive smell. I also noted that they weren't fanning their wings, which I would have expected if they were spreading an odor. I observed bees returning to the hive; they weren't attracted to the cluster (which was in the area where the feeder had been), and as a matter of fact, they completely ignored them, either barreling through or walking on their backs into the hive.
I peered through the opening and saw that the oddly acting cluster extended into the hive, on the screen bottom as well as the upper edge of the opening. Franklin Carrier had mentioned in his book the need to occasionally provide the bees with salt. Maybe they were getting minerals. I got a small amount of salt and put it in the middle of the cluster. They steered clear of the grains (which were all gone the next day).
At a loss to figure out what was going on, I turned to the internet. First search was for bees cleaning, since that is what it looked like they were doing. That led to washboarding. Aha!