Imagine the bees crowded around the screen where it looks like they are trying to get through the screen, at any cost, even if they have to shred themselves to do it. They will be biting and crowding so tight against the screen that its hard to imagine air being able to get past them. That is how I describe balling, it is similar to how they ball the queen if she weren't in a cage. The degree of this behavior can vary, being more or less intense depending on factors that only the bees know.
It is extremely difficult to remove the bees that are in this mode, without smoke.
That is a great explanation Joseph. Here's a picture that may help:
There is a queen in there under that ball, they had balled. The poor thing was piping away. I couln't take it any more and seperated her from them and she went down in between the frames.