updn means

«up, down, but mostly up»
in reference to the fact that any audio and control signal out there simply consists of various ways of going up and down, up and down, and up and down again. The art is to make it go up and down at the exact right time and rate. You may have noticed that the letter sequences 'up' and 'dn' are exactly point symmetric. That is why the updn logo often rotates: it only needs to go around halfway to become itself again. So you might interpret the logo to say 'up' twice, written around the edge of an imaginary circle.