Most of the HP's I've messed with move the cartridges immediately after they're turned on. Even if you turn it off immediately, it'll move the cartridges back over out of reach before actually shutting down.
So I turn them on with one hand on the plug in the outlet. As soon as the cartridges are within reach, I pull the plug out of the outlet. They can move pretty fast so it can take a couple of tries to get the timing right.
It's not very elegant but it works.
