Mask Changing (or Face Changing) is an ancient Chinese tradition in theater, used to indicate a character's shift in mood and demeanor. Accompanied by fan flourishes and other methods of obscuring the actor's face, masks are changed with lightning speed. He Hongqing is considered one of the best. Check out the video at BBC.
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His website is danchanmagic.com
The rest is reasonable engineering and well-practiced staging. Playing up the "secret" is playing up the least interesting part of the art.