The Speaker Resonance tab to me in my words is the way to dial in the symbiotic relationship between the power amp (transformer/output tubes/rectifier) to the speaker/cab *and* vice versa. They are a system, not independent. Small changes to the resonance has the effect of dialing in the amp/cab as a system...You will learn very fast what works, how it works...
My process is like on the video... bump the low end HZ slightly, listening. When you hear it 'blow up' on the low end, back it down. Then turn the low end resonance up till it blows up again, then slightly back off. Then change the "Q" and balance between the level and the Q till it is fat, but tight. Then on the high end just turn the level up slightly, a bit at a time... it'll start to explode on the top end, back off slightly. Done. Mess with the middle by turning it up a touch at a time... often this gets left at zero in my world. **Added note: these changes are absolutely tiny, small increments. The key is to trust your ear. When it sounds like "too much" trust that instinct and back it down. You might only end up a few thousands of a value from stock; but it will be worth it's weight in gold... trust me...
That's really the whole story. Hope that helps. If you want more detail on something, just ask.