Good topic, and not complex to understand.
You pack when your penis expands enough to fill the tube from side to side ... this comes, when the width is filled out first.
Usually it
packs first at the base, then at the head, and finally all along the shaft.
Once you fully pack, all the
pumping action is
directed lengthwise, enlarging the external penis, but aswell coaxing your suspensory ligament to stretch.
For example, with my girth of 7" if I wanted to target length pumping I would be wise to choose cylinder with a diameter of between 2" - 2.25".
During the session, the 7" girth would quickly "pack" the tube, and then the pumping action is concentrated on the length, as it has no option but to expand in that direction. Nothing complicated, no will it, or maybe, it will work like that.
In the past, great care would be needed to ensure when packing the tube, that the pressure didn't injure the glans [head] as the pressure becomes very concentreated around this area when packing, but, as you know, unless you've been in a cave for the past few weeks, MoS now sells the most potent aid for the PEer, and that goes for pumpers aswell ... the Silisleeve & Silicap
So, by wearing the Silicap when packing, you protect the glans from the negative pressure it would have otherwise been receiving.
In fact, using the sleeves MoS sells, also reduce fluid retention, so ensuring the actual pump session is as productive as can be.
Think of the sleeves as a protective scuba diving suit for your penis.