What to see?
At first I couldn’t see a thing, but after I concentrate for a moment I was able to spot two images: the image of an old woman and the image of a young woman. Both figures are present, but you will not be able to see them simultaneously. Now that you can see them both try to play with the images, taking turns.
What to do?
This type of reversible figure resembles a lot with the vase/figure illusion. This kind of reversible figure is important to see how the brain interprets the same static image depending on the main content fixate by the retina.
To find out how it works you have to explore the contours of the image and you will tend to remain stable when you will find a figure that corresponds to the pattern you have in your mind: the pattern of an old or a young woman. Some of the lines tend to favor one figure, others the opposite one. Even if certain lines are ambiguous, your perceptual change does not involve a figure/ground reversal.
How it works?
When you pay attention to different parts or contours of the image you tend to initiate a change of perception. Even if the new perception appears it’s not necessary to shift your gaze because that happens spontaneously. When you change your look on other lines of the image it’s clear that you also change the main object of your attention, the object of the perceptive field. All the other lines become part of the background and will remain like that until you will pay them attention.
Some of us can’t see both figures because they tend to leave in their mind just the idea of old or young. For example a young man will tend to see the young woman because it’s current interest.