What does a penis pump do?
It sucks bodily fluids into your penis by reducing the outside pressure. The fluids are mostly water that temporarily fills various cavities, but they also include blood that is drawn into the blood vessels that cause a normal erection. (A normal erection occurs when a small muscle relaxes, allowing more blood to flow into the penis.)
The net result is a temporary swelling that has many similarities to an erection but will subside slowly as the watery fluids return to where they came from. The blood will flow out more quickly. resulting in a swollen but limp organ for perhaps as long as 30 minutes.
It is probably more important to note what it does NOT do:
1) It does not lengthen the penis. Indeed, the urethra is the determinant component of penis length, and it is decidedly non-stretchy.
2) It does not result in any permanent size increase. The swelling is temporary and is more like a bruise or a bump on your head. Your body will "heal," and it will return to whatever its normal size is.
3) It does not stimulate growth. Your penis is not like a potted plant that will respond to fertilizer.
If you are determined to increase your penis size, there is one solution that is guaranteed to work: surgery. Modern surgical techniques will enable you to add several inches—assuming you can find a donor. OR perhaps several lesser donors, of various girths, so you can have the world’s first "ripple dick".