eli5: What does the puff of air machine at the eye doctor do?
The puff of air pushes on the surface of your eyeball, and a sensor measures the 'dent' made by the puff of air. This tells them how much pressure is in your eyeball.
(Poke a fully inflated basketball with your finger, the surface doesn't move. poke a deflated basketball with your finger and you can make a noticeable indentation)
It causes the cornea to "squish" slightly under the pressure, and how much of that squish occurs allows the machine to calculate the internal fluid pressure in the eyeball. It's not the most accurate measure around, but it's good enough to tell the difference between regular eye pressure and incipient glaucoma, and since there's no actual contact between the machine and the eyeball, it's an extremely safe means to measure pressure.