In theory, at least, they should cause new tissue growth in the penis.
However, penis extension devices require a high level of commitment to wearing them (3,000 hours+), and most of them are pretty uncomfortable.
On the upside, unlike the most common form of penis lengthening surgery (Ligamentolysis), a penis extender will increase both the flaccid and erect penis length.
While the theory supporting penis extension has some validity, in practice a significant permanent gain in length is almost never achieved.
Most users wear the penis extension device for a while (maybe a month or two), before concluding that the insignificant gains in length (if any), are not worth the inconvenience of wearing the device.
The key to getting results from a penis extender is finding one that is comfortable enough to allow you to wear it for the maximum time under tension possible.
Most of the time penis extenders are most irritating at the point the plastic loop goes around the glans or head of the penis. It is at this point that the pressure from the metal tension bars is exerted on the head in order to stretch the whole shaft.
Phallosan provides a solution to the irritation and tenderness experienced with extenders behind the head, by instead sealing the head in a vacuum which then transmits tension along the shaft via an elastic belt wrapped around the waist.