It wasn't dark yet, so I played around with it the dark garage, got used to using the on/off switch which seems to have two levels of intensity, and using the gain knob. Had fun with it there. There is a dial on the front by the eye cup, not sure what it does?
There are three positions on the switch: Off, On, and IR On.
The IR on turns on a built in IR illumination and this is used for making light in either pitch black or very low illumination conditions. When on, you should see a small red indicator at the edge of the field of the eyepiece (where will depend on the orientation, but it is right at the very edge and will require that your eye be close to the eyepiece to see).
A caution here is that if you accidentally turn on the illuminator while observing and only click it one position, the device will not be off, and if you leave it this way and uncapped, it can cause you to burn in an image to your display. In bright light, the tube is supposed to turn off in a minute, but in lower light with a very bright source like a street light, that won't happen, so you can damage your tube. Always make sure that you are off by looking into the device before you cap it for storage.
The tube can also appear to have different levels of illumination though you rarely see this in operation. For example, you might turn on the tube and see the display very bright, then qo dim. If the conditions are bright, the built in automatic brightness control (ABC) will automatically step the display down. It is doing this more or less constantly, but when you first turn on the tube, if it is bright in the environment, you will see the maximum step down intensity all at once. That is automatic though and is independent of gain. This would appear to change the charge voltage to the phosphor screen so that it does not emit as strongly as it would with a full blast of electrons from the microchannel plate. I do not believe that it changes the gain, only the brightness (or otherwise, wouldn't that call it automatic gain control??).
Make sure you double check power and cap the device when not in use. IF the cap is on it and you accidentally leave it on it will just run until the battery dies, but it will not hurt anything. Capping is just good insurance.
If you are using a PVS-14, put a piece of black electrical tape inside the cap over the training hole. Once again, if you leave it on, even though the training hole is tiny, it is an image intensifer, and if there is a bright image on the screen for 50 hours, it could damage the screen, and will essentially shorten the life of the tube by as much as 50 hours depending on how bright the light is. If it is in a case, then nothing will probably happen, but if it is sitting out somehwere in high ambient light, it will be essentially giving full wear to the phosphor screen.
IF you ever see Night Vision devices for sale on Ebay and the view in the picture shows that there are dark areas in the screen, that is a high wear screen and in time, even with good practices, the screen will eventually start to degrade (we are talking many many thousands of hours, and for astronomy, it is pretty safe bet that the tube will last a lifetime because this is a very low light environment vs looking at the cars coming by in front of your house to marvel at how bright the taillights are to NV (hint, they are as bright as the headlights, but your eye does not see them at this same brightness. A quick check will show that both car headlights and car taillights are both about 55 watts, but the NV sees read much better than the human eye.)
So, be careful that you are not in On mode when you store your device, and as insurance, cap it.