If you pump too fast the edge of your testicles can be sucked into the bottom of the tube, which creates a sensation similar to being kicked in the balls. Feeling faint is quite a mild way to describe that, and I wouldn't want it to continue for two minutes.
The SomaErect pump which I was prescribed had an insert tube to use for the first few times. It obviously limited the girth to which my penis could expand but ensured there was no space in which a bit of testis could get trapped. Once expansion of the penis diameter happened quickly enough, I could just use the main tube.