Pumping is something that is more often done to achieve bigger balls rather than longer ones but the principal is the same as other ball stretching techniques, only using a vacuum to stretch the skin, muscles and ligaments as opposed to gravity.

It should not be done for long periods of time as permanent damge can easily occur. It is also easy to burst small blood vessels if too much vacuum is created. Because of the nature of it therefore it should not generally be used as a front line standalone ball stretching method. It has its attractions however and has the ability to produce more stretch than conventional gravity methods and with less discomfort (if done in gentle moderation). The picture above for example shows around 7" of stretch which the user has difficulty achieving by other means.

Generally vacuum pumping would be followed by the use of more conventional weights or stretchers to maintain the stretch achieved.