It's the first attack, i.e., the first right-click. You can stand there and kill everything with the Arcane Dynamo powered beam. As soon as you release the right mouse button, the effect wears off. Note: due to this game mechanics, it also skips the Archon transition, which means the 75% do not apply to the Arcane Destruction, if you think about using the the explosion rune.
Don't get exactly what you're saying - but why not just switch source/weapon when you need APoC or not? Or why not just use a signature spell like Shock Pulse with Living Lightning rune, it works like a charm, I've been using it for a few weeks now... no need for APoC.
I get what he's saying, that a sig spell with dynamo would be pretty op. I second the statement about just quick swapping to an OH with APoC (and MH) if Archon is down. It works great for me and only takes a couple seconds to refresh it most of the time. I use a low dps MH with APoC to make it even easier to spam WW.