I find it too difficult to keep my dick aligned between the rollers in the vertical position. It works okay if I have some erection, but if I’m more flaccid it curves too much and I get my dick pinched in the PJ arms somehow.
So, I use the PJ with the rollers horizontal, and then as I do the strokes I twist the device a quarter turn to the right and left to hit the sides. It takes a slight erection and some practice to get it right, but I get a good jelq-pump that way.
I am, by the way, using a PJ that has foam athletic tape on the rollers, so they’re much smaller than usual. This allows me to get way down on the shaft towards my abdomen. I get a good grip there, and then do my strokes with the “rollers” never rolling; they stay positioned in the same place on my skin as I pull the PJ out from my body, and then relax the grip slightly, push back towards my abdomen, and then pull out again. You guys who PJ probably know the stroke. But the smaller rollers make it easier to emphasize different parts of the shaft.
One of my PJs is pictured.