Discipline applied by those who do the work that must be disciplined. ExtremeProgramming encourages InternalDiscipline. Programmers are expected to DoTheRightThing, and management doesn't run around with a big stick making them do it.
Compare to ExternalDiscipline
See also ProgrammingValueSystem
This is possibly important in light of the research suggesting that people are more motivated if they don't have external rewards.
How to MotivateProgrammers is discussed at http://www.fsf.org/philosophy/motivation.html (among other places), and mentioned in passing here on CitiesBackFromTheEdge. I'm sure there are some other good articles on IntrinsicVsExtrinsicMotivators? and MotivationVsPunishment?. UK XpDay? 2002 included a mention, but I forget the details.
TheReformSociety struck me as being the result of careful application of InternalDiscipline.