This article is from the Magic FAQ, by Paul Nielsen firstname.lastname@example.org with numerous contributions by others.
Since this is a public forum some of us would prefer not to openly
discuss the secrets behind the workings of magical effects. The
following are some of the reasons given to support this policy:
1) You ruin the illusion for others.
2) Magic as an art only exists when the performance appears impossible.
Focusing on "how it is done" reduces the art to a puzzle.
3) Magic as an art exists to entertain. To allow an audience to focus
on the performance and not the gimmick is the ENTIRE REASON for the
performance. The curiosity you feel when you're fooled by magic is
part of the entertainment. When the curiosity is removed, so is some
of the entertainment.
4) Although the public thinks it wants to know what we are doing they
are disappointed when they find out.
5) Some folks count on the elements of surprise and subtlety in their
techniques to feed their kids and pay rent.
6) The secrets are not yours to give. The creators of certain effects
have typically written books or produced effects in order to 'stake
their claim' and to be compensated for their work.