Singing in the mask is a weird way to describe this technique, but its the accepted terminology amongst singing teachers so I'll use it as well. I use the image of a masquerade mask as the mask the phrase refers to is this kind of mask. The short explanation of this sound is that you are adding a frequency to your voice that buzzes around your eyes, especially in the cheek bones or eyebrow areas.
The goal of singing in the mask is to fill out an already pleasant sound, not to fix any problems. It adds the highest frequencies to a sound, filling it out and balancing out the lows in the typical singer's voice. If you aren't sure what high frequency would sound like or how it would add, think of a really annoying whiney voice. The kind of voice that is shrill and piercing. By itself its quite unpleasant, but added in small amounts to a low sound it really does improve the quality.
My method of teaching how to sing in the mask is to focus on creating exclusively a mask sound first. The extremely annoying and piercing sound. Think of the vowel "a" as used in the word "and." this vowel has two sounds, "a and uh." We want to divide this into only the first sound and sing that. Singing a note that is medium high in your range, sing this sound in the most tongue forward, aggressively annoying, and nose wrinkled way you can. The more piercingly irritating it is the better. When you make this sound correctly, it should immediately become incredibly loud and you will probably feel a buzziness in the "mask." This is how you create a mask sound. The next step is to add it into your regular voice.
Now that you can sing in the mask, try singing normally and slowly adding in this annoying sound. Pretty quickly in the process you should notice a full sound followed by a shrill annoying sound. The full sound is what you're shooting for. If you don't get the full sound, you've added too much too fast. The correct quantity of mask singing usually sits around 5% of what you'd have felt with the annoying sound. Its really quite minimal. With a shockingly low quantity of the nosey sound, you're voice will sound much more filled out.
THINGS TO BE CAREFUL OF:
When singing in the mask, you should be just as successful with your nose plugged as with it unplugged. If you cannot create the sound you think to be correct with your nose plugged, you are simply singing nasal and not in the mask.
Record yourself. Chances are the quantity that sound correct to your ear may not be the correct amount. You may need either more or less. For myself, I need less than I think I do.
Don't replace low frequency with high. If you add mask singing, add it to low sounds. Do not replace the low sounds with the high sounds unless you are trying to belt way beyond your range. Even then, be careful with it.
Hopefully this helps you find your version of singing in the mask. If the method I provide above doesn't seem to be working for you, there are several other approaches that would make this article much too long. If you need that extra assistance, sign up for a voice lesson with me by clicking here!