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.