I've known some stuff about trusted platform modules for over six or seven years. I understand their usage in (un)wrapping keys and storing information in NVRAM which may then be locked permanently or until the next power cycle. And though I understand how the platform configuration registers are updated (their current value and the input value are hashed together), I do not understand what good they do or how they can be used by the firmware or software to detect changes.
If the TPM is supposed to be a dumb (passive) chip, then shouldn't it be incapable of determining whether or not to deny access to the data it holds based on the PCRs' values? If the PCRs are used to compare two states, then how can the system software do this if it needs to know either past future data that it would seem it cannot ever have access to? I am most familiar with the Integrity Measurement Architecture, so maybe you want to use that to explain this stuff to me.