The man holding on to the climber's rope runs to shorten the fall. He manages to prevent him hitting the ground, but can't stop a bone-crunching impact between the climber's leg and the rock as the rope comes tight. Blood flows.
Moments like this form the heart of a new film, Hard Grit, which for the first time captures the essence of the British spirit of climbing: the readiness to risk your neck to get to the top of a route. Some may find it hard to stomach: Channel 4's Big Breakfast thought that its viewers would find the above sequence "too upsetting", and cut it from its coverage last Wednesday of extreme sports. Even the film's makers, Richard Heap and Mark Turnbull, found their year's work almost unbearable: the climbers are all personal friends.
Yet Everest it ain't. Few of the 20-odd routes shown in Hard Grit are more than 100 feet high, and all are on the gritstone edges around Sheffield. But whereas an averagely fit businessman (or woman) can hire guides who will get them up and down Everest, the average climber couldn't get off the ground on these routes. And if they could, they would regret it - the consequences of falling off are terrible, the obstacles to success huge.
To find the people who can and do take on what Richard calls the "calculated insanity" of these climbs, you have to go to Britain's most daredevil postcode: S7.
For a postman, S7 is a quiet suburban part of the western edge of Sheffield, stacked with two-up two-down terrace houses. A three-bedroom house here costs about pounds 42,000, or you can rent a room for about pounds 30 a week. The city is enjoying a renaissance: cafes and restaurants are opening at a frantic pace, and jobs are beginning to pop up as service-based businesses latch on to a location that is cheaper than London yet within easy reach of motorways.
For a climber, S7 is the perfect location - full of houses with cellars (which make ideal indoor training grounds during wet weather), 10 minutes' drive from the gritstone edges, and, best of all, populated by other climbers.
Many originally come to the city as students and never leave: they graduate, get jobs in the area and carry on climbing. As students, they naturally seek out the cheapest and most convenient parts of the city.
"It used to be S11, but people just started moving to S7 about three or four years ago, often after being students," says Neil Pearsons of the climbing magazine On the Edge. "Their friends would visit and say 'This is nice' and move in too."
Naturally competitive, the best in the field push each other to try harder and harder climbs. That usually ends up with someone standing at the base of a smooth expanse of gritstone - the gritty rock peculiar to the area - trying to psych himself up to do something that could kill him. "I think it's a bit like the kids in the estates playing chicken with cars," muses Richard.
One man who has found himself in that position repeatedly is David "Seb" Grieve, 33, a post-doctoral researcher in materials science at Sheffield University. Like many other working climbers, he has turned down more lucrative jobs elsewhere because they would interfere with his climbing.
"Before you do those routes, you ask yourself why. It's horrible, the feeling," he says. "The longer you spend preparing and practising, the more scared you get. Sometimes to get the courage you just have to start climbing it anyway."
Richard filmed him attempting one route, called Parthian Shot, which had gone untried for eight years because nobody thought you would live if you fell off. Seb fell off. He lived.
Neil Bentley, a friend of Seb's who has done comparable routes, says: "While you're there, you don't notice anything else. You don't feel scared. Often you do the hardest part of the route - then you sort of wake up. That's when you get scared."
So why do it? "I think it recaptures something of the first day you ever climb," suggests Richard (who has himself been cajoled into doing dangerous routes).
"The first time you climb, it's all new and terrifying. But afterwards, it's wonderful. I think you try to get that feeling back."
"Some routes look so beautiful you're compelled to do them, because you know you can," says Seb. "It's not a particularly pleasant experience before you start. But afterwards, and actually during, it's good. These routes are part of climbing history. You want to be in it."And, for now, not in the obituaries column.
'Hard Grit' can be ordered for pounds 12.50 from Slackjaw, 60 Wath Rd, Sheffield S7 1HEReuse content