MOTM

Hi,
 I've just built my first batch of MOTM modules.  They were fun to 
build and worked right off the bat.  I am confused about the 
function of the Bias switch on the MOTM-830 dual mode mixer.  
What does it mean that it sends DC voltage?  How do I use that?  
What is it good for?   

 Thanks.  I'm still green as far modular synths go!