a common reference to the action and state of mind when one is either street racing in their automobile, or ont the track. the physical meaning is reference to the pigment of ones knuckles fading from holding the steering wheel tightly. the mental reference is due to the edgy state of mind one is in, when in a similar situation.
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.
Awaiting an outcome, or termination, of something with almost unbearable anticipation (as in clenching one's fists tightly to get through the agonizing final days or moments of waiting for something to happen or be over with)
Brooke's mom was told she couldn't have a drink for 30 days before the wedding, and she knew she would have to white-knuckle it!