This is quite a well known illusion. Basically, a hollow mould taken from a face actually appears to be a three dimensional convex face, even though it is actually concave.
Tim has got hold of this amazing hollow face of Richard Gregory, Professor of Illusions at Bristol University, which he demonstrates in the video. It is twice life size, and has been painted as well.
The Hollow Face Illusion is also demonstrated in the Einstein Mask, and also by the Dragon Illusion.