Can someone explain this?

Your eyeballs technically shut off for a fraction of a second when they're moving. Look at a spinning ceiling fan then quickly look away.
You'll notice that the fan appears still for an instant when you dart your eyes away. This is the last image your brain processes before your eyes refocus.

The flicker rate on some LED bulbs might have an exaggerated effect.

>>56630162
Ah ok. thx.

This effect also appears when i move the led
( and my eyes don't move)

>>56630126
It's an effect coming from the saccades (the flicking movement of your eyes), combined with the way your brain fills in images while your eyes are moving, and the refresh rate of the bulb (it's flashing at 50hz if you're in the US, 60hz elsewhere). Sometimes the flashing is more noticeable with LEDs than incandescents because the incandescents still glow a bit inbetween electrical pulses so it smooths out the effect while LED bulbs turn on and off much quicker.

>>56630162
that happens when under flickering artificial light
the only reason the fan appears blurry when you stare at is is due to persistence of vision, when you move your eyes away quickly, you will see multiple near-still "flashes" of the fan, not because of anything your eye or brain is doing, but because of the strobing of the artificial light

>>56630283
>>56630463
and yes, LEDs turn on and off far faster than most other types of lights (incandescent, CCFL, halogen, etc), so they have less persistence themselves, making the effect more apparent

this does not apply to lights which are on constantly (such as when powered with continuous DC power, such as from a battery)