Perhaps we should say more accurately; with a double-blink of the eye. The soft contact lenses are capable of measuring what's called the electrooculographic signal of each eye, and can in turn use those signals to manipulate the contact lens. According to Shengqiang Cai, who serves as the lead researcher on the project, electric signals are generated from specific eye movements (such as looking up/down or left/right).
In this instance, the researchers are using the specific electrical response of a double-blink to allow the lens to change its focal length. This is done by stretching the soft lens material -- like a muscle -- to become more convex, thus zooming in.
"The motion and deformation of the soft lens are achieved by the actuation of different areas of dielectric elastomer films, mimicking the working mechanisms of the eyes of human and most mammals, writes the research team in their scientific paper.
“Even if your eye cannot see anything, many people can still move their eyeball and generate this electrooculographic signal," added Cai.
At this point, the researchers are confident that in the near future they will be able to create prosthetic eyes that can help visually impaired patients to see better or even adjustable eye glasses that can change their zoom level on command. You can find the full context of the UCSD research, here.