These devices work so well because they use traction. This refers to the stretching of tissues in the body and it is effective when it is done gradually over time. Pro Extender A good penis extender will apply the right amount of outward pressure so that the cells in the corpora cavernosa to duplicate. That allows more blood to flow into the penis when a man becomes aroused and it will increase erections in terms of both length and thickness. Having the cells duplicate in the corpora cavernosa will also mean that the penis is larger when it is flaccid.