A penis pump is a cylinder that is fitted over the penis, with a manual or motorized pump to create suction. As the apparatus creates a partial vacuum around the penis, blood is drawn into the penis, helping it to become engorged. As vacuum increases, the difference between the inner blood pressure and the pump pressure increases as well; excessive pressure causes vascular damage rather than a harder penis.
Penis pumps, usually described in this context as vacuum pumps, have use in conventional alleviation of impotence. The pump itself is essentially as described above, although often made to higher standards of quality with a much higher price, and arrangements for distribution by suppliers of medical equipment. The purpose is not to cure the condition, but to attain an erection by mechanical and hydrodynamical means even if there is nerve and vascular damage preventing a full natural erection; once the penis is erect, a compression ring is slid on to maintain the erection which will otherwise subside immediately.