I caught the last half of last night's Good Eats, wherein
Alton Brown explains the physical chemistry of the braise.

Dissolve the collagen in low heat over a long time until the
meat is falling-apart tender.

Let the meat rest and the collagen reforms into gelatin,
which is not as tough as collagen but takes a higher heat
to dissolve so it remains stable.

A tough piece of meat is made tender and solid at
the same time.

--Blair