Neckflash (verb): To neckflash, is to be in such a hysterical laughter fit that you lift your head back and flash your neck to the world. The average neckflash only lasts for about 5 seconds, however the world record is at about 6 seconds.
When your neck gets really sore from sitting in bed all day/night watching Netflix on your laptop. Its caused by the awkward positions you sit in so you can see the screen while trying to binge watch your favorite shows on netflix. The only way to try an prevent it is to constantly change positions.