This trick isn't documented in the 2006 TSX Owner's Manual. It's a useful function if you're driving late at night and want to minimize display lights that might distract you. You can also turn the select/reset instrument panel light to its lowest setting to dim the dashboard background lights, courtesy lights, and of course instrument display.
All the functions and controls of the audio system and climate continue to operate normally, you simply don't have the clock, radio, and climate control displays.
:Press and hold the CD/AUX button on the base model stereo head unit for 10 seconds.
A beep will sound.
The clock, radio display, and heating/cooling displays turn off. Console backlighting remains on. Their functions are still active: you can change channels, adjust climate control. The displays are simply disabled.