Doepfer

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.