I was using one yesterday to fix a broken taillight/ license plate bracket, and made a few observations. PSI around 5, let it heat up a little first. Heat the rod before you start to heat the plastic to be welded. Apply the heat to the plastic, and as it starts to melt, apply the rod. Keep the heat moving around all sides of the rod, and keep heating the plastic ahead of the rod. Every little bit, as the weld you have layed out is still hot, take a metal tool, and press the weld into the surrounding plastic. We were welding something that required a lot of pressure to hold it together, and some funky angles, so one would weld, while the other held the piece together. The holder would then press the weld into the plastic, we were using a pair of side cutter pliers, which we also used to cut the rod at the end of each weld. This worked very well, as trying to pull the rod off pulls the weld back off while it was hot.
It works pretty well, the trick is definitely to get the feel for the amount of heat needed to melt everything together, without melting too much.