The big ring simply indicates how much extra power you've accumulated (just as the percentage right below it). It gets bigger every 20%.
The pale circle within the ring is some glowing FX that starts as a tiny dot at the center of the ring, expands, re-initiates and expands again. It completes this cycle faster when you mash the Z key quicker. Fancy graphics, you know.
And the Z key Picture on the bottom left indicates that you have to press the key shown. When using Moves, this Picture starts its animation in synch with the "square" that will indicate when to press the key to lend the next hit. But here the Z Picture animation keeps running, meaning that you have to mash the key.