The illusion works because the deer immediately grabs your attention. Once your brain identifies the animal, the surrounding branches, shadows, and tree trunks are automatically interpreted as normal parts of the forest.
To spot the hidden face, try ignoring the deer and look at the darker spaces between the trees. Search for separate facial features such as an eye, nose, forehead, or mouth instead of looking for a complete face.
Moving farther away from the image or making it smaller can also help. Once your brain finally recognizes the hidden man, the face often becomes surprisingly difficult to unsee.
