In a surround processor or receiver, THX is circuitry built into the device. This circuitry has four steps it takes in addition to the steps a normal surround receiver takes. Before entering the THX circuitry the input goes through the 5.1 channel decoder and enters the circuit as your L, C, R, SR, SL, and sub signals.

First is bass management. All speakers will be crossed over at 80 Hz and everything below goes to the sub.

Second is the re-equalization of the front three channels. This is done due to home theaters sounding brighter than a commercial theater due to the size difference, and the proximity to the speakers.

Third is timbre match of the surround speakers, which helps to even out the sound pans front to back and left to right.

The last step is decorrelation. This helps a home theater room sound much more spacious.

After the THX circuit, the signals go through the time synchronization circuit (i.e.- the distance setting when setting up your receiver).

In the case of speakers and some other electronics you are correct, THX is a badge, but receivers and processors must have added components to get approval.