I'm not endorsing it however if I recall the forms I've seen over the years independently bypassed different things, by different names by brand (so don't assume the terminolgy is "universal across brands), for example:
- video circuits killed/bypassed. Only audio remains.
- front panel fluorescent display killed [I used to think that was bunk until one day I noticed on a product that it really did remove a slight, faint buzz noise, at least for that product (it was a DAT recorder I owned)]
- "Source direct" = no tone control circuits engaged (they do sometimes inject faint, usually innocuous noise). The selected source goes straight to the amplification stage.
- "CD Direct" = no input selector function for you!. It's bypassed. The CD input goes straight to the amp
- "Super duper PURE direct" no ADC for analog incoming sources hence no digital room correction processing for you! [And for most units no subwoofer signal generation from 2.0 analog sources.]
- "HT bypass" usually means no volume control action either. The incoming preamp analog L/R signal passes straight out to the preamp outs without passing through any electronics other than that A/B switch itself.