Distance

If the jelking works through a penis, the stretch is much easier to exercise. Analogize the retraction of a member with a stretch of the rubber. When you stretch the rubber the first few times, it returns to its normal state, but if you pull the rubber all the time, it'll eventually be longer. The same thing happens to your member: the penis increases in size at multiple, persistent lengths.

The length may be one of the oldest exercises for a member, and it is indispensable for someone who wants to add in length.