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After the hidden figure has been recognized, returning completely to the original interpretation can become difficult. The viewer already knows which branches and shadows belong together in the second pattern. Instead of seeing several unrelated pieces of forest, the mind continues grouping those elements into the newly discovered face. The same visual information is still present, but previous recognition influences what becomes noticeable next.
This does not mean the viewer has discovered an image that was physically invisible before. Every relevant line, shadow and patch of snow was visible from the beginning. What changed was the organization of those features. Initially, the visual system grouped them primarily as elements of a winter landscape. Later, some of the same features were grouped as components of a human face. Hidden-image puzzles take advantage of precisely this possibility: one arrangement can support more than one meaningful interpretation.
The effect also demonstrates why context is so powerful in perception. When an image is introduced as a winter forest containing a deer, viewers naturally expect to find forest-related objects. Trees are interpreted as trees, snow as snow and branches as branches. If someone instead announces that a face is hidden in the picture, the viewer receives a new expectation and begins searching for face-like arrangements. A simple instruction can therefore change what receives attention even though it does not alter a single pixel.
Hints work for the same reason. Telling someone exactly where the hidden face is located can make the puzzle almost trivial because attention immediately moves toward the relevant region. A subtler hint, such as suggesting that the person examine the spaces between the trees, preserves more of the challenge. It changes the search strategy without directly revealing the answer. Once the appropriate region receives enough attention, recognition may occur naturally.