What's the meaning of "white knuckles" in this sentence?
" With white knuckles and a pink helmet, the tour began."
I read this sentence in an article from the Economist. In dictionary, knuckle means "A part of a finger at a joint where the bone is near the surface, especially where the finger joins the hand". But I don't see the point of mentioning a part of finger in this sentence. Can someone help me to figure it out?The whole paragraph in the article:
"A terrible threat stalks the streets of Washington,DC: unlicensed tour guides. These brazen lawbreakers imperil the public by showing them around the nation's capital without a permit. Your correspondent went undercover to observe at first hand the dangers tourists face in their clutches. It was harrowing. First, your correspondent had to balance on a Segway, a two-wheeled vehicle from which she could have fallen several inches to the cold, hard pavement. “Just try to relax,” purred Bill Main, the outlaw guide, “It's easy.” WITH WHITE KNUCKLES AND A PINK HELMET, THE TOUR BEGAN."

Question

What's the meaning of "white knuckles" in this sentence?
" With white knuckles and a pink helmet, the tour began."
I read this sentence in an article from the Economist. In dictionary, knuckle means "A part of a finger at a joint where the bone is near the surface, especially where the finger joins the hand". But I don't see the point of mentioning a  part of finger in this sentence. Can someone help me to figure it out?The whole paragraph in the article: 
"A terrible threat stalks the streets of Washington,DC: unlicensed tour guides. These brazen lawbreakers imperil the public by showing them around the nation's capital without a permit. Your correspondent went undercover to observe at first hand the dangers tourists face in their clutches. It was harrowing. First, your correspondent had to balance on a Segway, a two-wheeled vehicle from which she could have fallen several inches to the cold, hard pavement. “Just try to relax,” purred Bill Main, the outlaw guide, “It's easy.” WITH WHITE KNUCKLES AND A PINK HELMET, THE TOUR BEGAN."

Answer

Segways don't go very fast, so the writer is exaggerating the "danger". The helmet was literally pink. (Yes, we also have a rule that you must wear a helmet while riding a segway.)

If you grip something strongly (usually out of fear or excitement), your knuckles literally look white. We use "white knuckles" to mean an exciting or terrifying ride.