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.
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.