Mask gives us tools which can help us step past our conceptions of identity so that we can find something unexpected in ourselves.
Oscar Wilde said that if you wanted to hear a man tell you the truth, you give him a mask. I suspect that trance masks operate, in part, on similar principles.
Mechanically speaking, you've probably got a rough sense of how masks operate. You get a performer into a suitably "loose" and playful state, give them a mask, and see who they become. Internally, the process is more complex. There's a lot of discussions about altered states and possession in the work, but really it's just allowing these pieces of paper mache to offer permission to act like something other than yourself.
In many cultures, the masks are tools of shamans and mystics. With them, their rituals were permitted to expand. They could transgress rules or cross boundaries in order to tell their stories. What we call Trance Mask has "rules" for process and an intended "for theater" context. Simultaneously, it's a little more organic and anarchic than a lot of theater play. While there's definitely a method to it, you have to be open to finding out what happens when the masks show up.