[comdpa]

... I don't seem to quite understand this whole right-shoulder-as-a-backstop concept.

Have you ever jumped off the ground?
If you have, then the ground is your backstop.

Because the hitter uses the angle of approach procedure, the right shoulder does not act like a fly wheel in the case of a swinger...it acts like a backstop.

The feeling is that the right arm blasts off against the backstop that the right shoulder provides.