The passive TPMS systems that worked off the ABS sensors are first generation archaic obsolete crap. Those systems looked for differences in rotational speed among tires based on the theory that an underinflated tire would have a slightly smaller diameter than a properly inflated one and therefore rotate faster. Too many things can affect the accuracy of that type of system, and they are no longer implemented in modern cars.
TPMS is a very interesting, and deceptively complex topic and the technology is really in its infancy at this point in time. There's many potential problems that still need to be adressed, and criticisms such as the 25% underinflation threshold being too great. Here's a link to an online tire industry trade journal quickly touching on just some of the issues...
http://www.moderntiredealer.com/t_pr...t&storyID=1290
I'd personally like to see a user programmable TPMS system allowing you to at least set the trigger point to your needs/preference.