Understanding MDC (Modification Detection Code)

[MindFuq]

The faq states that having key preferences of TwoFish and AES implies
the keyholder has the capability of using MDC encryption.  This may be
true, but my tests are showing that MDC is disjoint from those
algorithms.  PGP 6.5.1i can handle MDC, and it's limited to the IDEA,
CAST, and 3DES ciphers.

How exactly does MDC work?  I know with MDC out of the picture, if
someone changes the ciphertext, the receiver knows.  Either the
receiver will get garbage, or the receiver won't be able to decrypt
the message at all.  So what's the purpose of MDC?

Also, I'm curious as to why PGP 6.5.8 (domestic) cannot handle MDC,
but PGP 6.5.1i can.  Was MDC capability removed, and then re-added in
PGP7?