It could not be much simpler. It has a small but mighty clip on the back, so you can just clip it to the edge of your hat next to your ear. There's an on-off switch on the back. There's no volume control.
The unit beeps to let you know when you are in rising air (i.e. air pressure dropping). The rate of the beep varies with the rate of pressure change. It is silent in sink (pressure rising). It is so sensitive that it changes if I hold it in my hand and raise it over my head. There's a very tiny adjustment screw, in case the beep rate is too fast or slow, but I've never had to use it.
I originally bought the Mini Vario when I became aware of the safety benefits of audio varios, and I was flying a lot in Blaniks with no electric devices at all. I still use it as a backup or a cross-checking device.
One of our ships has an audio vario system but the static plumbing is sometimes flaky. We have not been able to find the problem, but I suspect it has to do with moisture in the lines. If I don't believe what the vario is telling me compared to the altimeter changes or the feel of the air, I turn on the Mini Vario to break the tie.
I really believe in having backups for systems, because they have saved my neck a few times. In my first contest - one of my first cross-county flights - the battery totally failed, and that ship only has an electric vario. So the Mini Vario really saved the day.
My only complaint is I wish it were a little louder, or had a volume adjustment. If the wind noise in the glider is loud, it can be a little hard to hear. Other than that, it's a terrific little device and I almost never fly without it!