A reader writes (thanks nick!)
Not gonna impress any girls with this one, but… I was looking at my mother’s ceiling fan the other day trying to determine how many blades it had. It was on its highest setting so it was nearly impossible to do. Until I blinked. If you blink rapidly, it disrupts the brains attempts at connecting frames of sight into continuous motion. Thus a whirling blur becomes a clear frame of sight, easily analyzed. Not sure where else this little trick could pay off. A nice illustration of the characteristics of our visual systems though.
Cool. Freed from the constraint having to make sense of continous input, your visual system can to make sense of the single ‘frame’ of input it does have. An example of less is more? I noticed something similar when riding my bike. When I glance down at the front wheel, it appears blurred. But when I look back at the road, my visual system delivers me a snapshot of the wheel, unblurred. What is happening – I’m guessing – is that as I move from looking at the wheel to the road ahead there is a moment of saccadic suppression [Hack #17] when visual input is cut off. Into this gap the ‘frame’ of the wheel is resolved. Also lending a hand may be a neural mechanism which turns off saccadic suppression if the velocity of the eyes matches that of a moving object (with your eyes stationary a moving object is blurred, with your eyes moving a stationary object is blurred, but if your eyes move at the same speed as an object you can get a clear image). For this to work the object needs to be nicely textured, so your low-level visual apparatus can gauge its velocity. Which explains why i get the effect on my mountain bike, which has big treads on the tyres, but not on a road bike, which has smooth tyres.
Mind Hacks 
Try this; When sitting by a computer monitor (CRT), take a bite off a piece of hard bread, hard snack or similar. While chewing, look at the monitor indirectly (corner of your eye). You will notice that the picture will start flickering wildly. This will also work on some TV:s. I guess the crunching during chewing makes the light coming into the eye move very quickly in different directions than when the eye is relatively still.
If you got a really crunchy piece, it will flicker even when you look straight at the monitor/TV.
If you don’t have anything to crunch you can get the same effect by making a “BRRRR…” sound with only your lips. Not recommended if you are surrounded by colleagues who might think you have become crazy. 🙂
Isn’t this an optical effect rather than a cognitive effect? It’s the stagecoach wheel phenomenon, in which the wheels of a stagecoach in old westerns appear to be turning backwards because each film frame has photographed the wheels at positions slightly *behind* their positions in the previous frame. If each blink coincides with the fan blades being in the same position, then the fan will look like it’s standing still because you are only looking at the blades when they happen to be in the same spot.