So a much simpler and concise explanation from what I am understanding is that the slip ball indicates how far off the precived direction of gravity is. Centered would feel like gravity is going straight down relative to you, and off to either side means that it feels like you are tilted to either side.
Good video but where is the old version of the same topic which has more contents and explanation? I really want to see that one because it's more helpful to understand this instrument in detail and the concept of relative aerodynamics more intuitively. Hope you didn't delete it and upload again..