When we talked about Rest Stroke Alternation , the first thing we did was create a physical point of reference by placing "p" on the 6th string.  Since we are now discussing how to use "p" as a playing finger, we need to establish a different physical point of reference, in this case with "i" on the first string.

There are only two things you need to know about the Free Stroke with "p": The before picture and the after picture.  And here they are:

Before:

After:

It is very important that "p" follows through to the "i" finger during every Free Stroke.  It is actually more efficient to have this "extra" movement in the same way it is more efficient to allow the follow-through of a golf club or baseball bat.  In very simplistic terms, your body has two types of muscles:  muscles which open joints (Extensors) and muscles which close joints (Flexors).  Your triceps (Extensors) open your elbow and your biceps (Flexors) close your elbow.  In the same way, an Extensor opens your thumb in preparation for a Free Stroke and a Flexor closes it in the execution of the Free Stroke.  Stopping the Free Stroke before the full follow-through requires the Extensor to act as a brake against the force of the Flexor.  It is much more efficient to allow the full follow-through of the thumb before drawing it back with the Extensor.