When one is driving and has to piss so bad that they are squeezing the steering wheel so hard that there knuckles are white.
I was white knuckling for 15 minutes this afternoon when I was stuck in traffic.
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.
Jesus, I'm sweating, I was white knuckling my way through that interview. Those guys are scary.