Noob player, noob Sona power chord question

I understand that by using any three normal spells (Q,W or E) will proc power chord. I understand that the power chord buff is dependent on the last spell cast that proc power chord.

What I am confused about is, once power chord procs, and I see the colored ring around sona what is the most optimal way to actually use power chord ( or maybe the only way)? Is it auto attack only buff? Or do I use Q to proc the blue ring and then once Q is off cool down use Q again for a more powerful " power chord Q"?

I have played a couple of games with her and it still isn't apparent.

The tool tip says next " basic attack" not auto attack, so I am a bit confused.

Thank you in advance for helping a new player.

-T