When you're taking a situation as it comes and flying by the seat of your pants. Often attributed to anxiety, and feelings either good or bad. A reference to clinging fiercely to something, such as a rope, to the point where ones knuckles would turn white from forcing blood from the capillaries.

When you eat dirt for the first time and leave your family to build a shack in the woods

When one is driving and has to piss so bad that they are squeezing the steering wheel so hard that there knuckles are white.

When you maturbate so hard your knuckles are white from holding your dick too tight.