| (Headroom eXpansion) Dolby HX Pro is integrated into a tape deck's recording circuits and modulates the bias recording signal, lowering it when high frequencies are louder.
The bias signal is necessary to help the magnetic tape store the sound waves as magnetic pulses properly, it is an ultrasonic wave similar to a carrier. As the dynamic range increases, the combined sound+bias may saturate the tape (distortion happens). Therefore the HX circuitry eases off on the bias to leave more headroom for the high frequencies of the audio material, themselves acting as a substitute bias signal.
Source: http://hyperphysics.phy-astr.gsu.edu/hbase/audio/tape.html | 
