The crystals on the screen refresh at a certain ms rating (8, 16, 12, ect) if they refresh to slow (generally 16 and above) then you will see an afterimage since the crystals do not change fast enough, it is almost like a trail following whatever image is ghosting.
Is that clear?
Basically the crystals go so slow that the newer ones start showing an image before the older ones clear the image.
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