If you are nearsighted (like me), you may have noticed that if you incline your glasses, you can see distant objects more clear than with normally-positioned glasses. If you already see completely clear, you can distance your glasses a little more from your eyes and then do the stuff. To do so, rotate the temples while keeping the nosepads fixed on your nose, as is shown in the figures.
As I said, starting with your glasses farther than normal from your eyes, you can observe the effect for near objects too. (By distant, I mean more than 10 meters and by near I mean where you can't see clear without glasses)
Note that if you rotate more than enough, it will distort the light completely. Start from a small $\theta$ and increase it until you see blurry, distant objects more clear. (You should be able to observe this at $\theta\approx20^\circ $ or maybe a little more)
When looking at distant objects, light rays that encounter lenses are parallel, and it seems the effect happens because of oblique incidence of light with lenses:
The optical effect of oblique incidence for convex lenses is called coma, and is shown here (from Wikipedia):
I am looking for an explanation of how this effect for concave lenses (that are used for nearsightedness) causes to see better.
One last point: It seems they use plano-concave or convexo-concave lenses (yellowed lenses below) for glasses instead of biconcave ones.