Hey Jim,
Real quick because I just got home from a ride. With the PVL I never do a half pull (not even sure what that would be), you need to pull the lever handle all the way down in order to 'release water into the basket'. I hold it in the lever handle down position for an amount of seconds to be determined by the coffee, the grind, the dose, what I see and how I feel. Sometimes I hole fully down until I see drops start to fall into the cup, sometimes I'll slowly release the lever before drops fall and as the lever rises up the drops will start to fall into the cup. At that point I do another full pull of the lever handle all the way to its bottom position and release the lever smoothly so it can naturally rise up on its own. At some point in its upward travel I will do another full pull down and again slowly let it move back up naturally on its own.
So to recap, I do a preinfusion pull, then I do two shot volume pulls.
At what point I do the pulls is really dependent on what I see in the stream and cup and what I've trended out from previous shots with the coffee I'm working with. That's the beauty of the lever, you can create any pressure profile you want and 'interfere' with the profile at any time you want.
It's easier to demonstrate than explain, so when you get your machine and you've pulled a few shots and you are ready to start figuring things out just speak up and I'll shoot a video or two for you.