Seems to defy logic. I would think that the water pressure coming out of your tub spout is independent of what kind of shower head (or no shower head).
As far as pulling a vacuum, there's a physical law that can account for that. Not sure which one. But it reminds me of chemistry lab, when we used vacuum filtration to purify our products. We turned on the water faucet and inserted one end of a rubber tube into the nipple attached to our Buchner funnel and the other end to a nipple attached to an aspirator connected to the faucet. We would then pour our product slurry onto some filter paper on top of the funnel. All the liquid would get sucked down into the flask leaving the crystals behind in the filter paper.
I did some research, and the physical law is called the Venturi Effect.