We use masks while freediving. As we go deeper, the air inside the mask - like our sinuses and ears - it gets compressed due to the increasing pressure and it's volume decreases. For this reason, you need to equalize the mask. To equalize the mask, we release some of the air we store in our lungs through our nose. The less air we use to equalize the mask, the better, and that's why we use low volume masks.
If you cannot equalize your mask properly, the compressed air inside the mask will exert pressure on our eyes with the vacuum effect. It is inconvenient for our eyes that it increases too much and reaches an excessive level. It can lead to eye and face injuries. If you have trouble balancing the mask, just stop diving and back to the surface.
To equalize the mask, blow a small amount of air from your nose into the mask. Do this with neither too much nor too little air while the mask equalizes. If you release too much air, some of it will escape from the mask unnecessarily. In freediving, we use our own air and air is our most valuable resource. Have your instructor or friend check that you are not losing air while equalizing.
As we start to accent, the air volume in the mask begins to expand. You see it as air escaping from a diver's mask. If you are fully comfortable other aspects of your dive, you can take back the expanding air in the mask as if you were going up. Try to sniff.
The deeper you go, the more flexible masks adapt to the shape of your face and help you use less air. I recommend using a high quality and low-volume masks for freediving.