You ideally run one oscillator into it. Whatever the frequency is that
goes in you get a square wave that's half, a quarter, an 8th and a
16th of the frequency coming out with adjustable levels. In musical
terms you get 1, 2, 3 and 4 octaves under separately adjustable. So it
basically synthesizes new bass frequencies you can balance the levels
of.

Historically you saw the /2 on a number of vintage synths being called
a sub-oscillator, mostly synths with only a single oscillator. It
gives you a bigger, deeper sound than a single oscillator -- though is
a lot less flexible than adding an expensive second VCO. With a real
VCO you can detune and modulate it to potentially sound even bigger
and use any wave it can produce rather than a square. Then again with
the A-115 you'd potentially need 4 VCOs to do what it does in a
limited way.

If you give it an ambiguous frequency, for example a mix of sounds or
a percussion sound you'll get something odd but potentially usable
since it's going to be a lower frequency made up of square waves.