If you play with it, you'll learn that it only drops very few psi... basically you put the large piece on, then thread in the inner (smaller) piece. Then pump away, or check, etc.. whatever you are doing. Then when you are done, you loosen the smaller inner piece and then open up the larger piece.
One warning, my friend purchased the same pump from PP after I did, and it looks a little different. Nothing bad, but not exact like the photo. The gauge was a changed slightly.
