The whir of the engine followed by the ground-shaking crunch sounds like someone revved a motorcycle and rammed into a wall at 90 mph, the kind of noise that sends chills down the spine if you're not paying attention.

Visually, it's as if your brain can't comprehend what's it's processing, the sight of a 500-pound machine doing a backflip causing the neurons to fire uncontrollably. Might as well be a Jeep doing a backflip as far as your brain's concerned.

It doesn't even feel right.

"The backflip is the most intimidating thing you can do," rider Chris Burandt said. "Everything you're doing, your mind says is wrong."

Freestyle snowmobiling made its debut at the Winter X Games a year ago, shocking fans and even athletes in other action sports by seeming to defy physics.

A year later, it's still causing sensory overload.

Flying off manmade ramps and snowpacked jumps, the riders gun the throttle hard and pull harder, somehow getting those massive machines to whirl around and land back upright.

Fans stack dozens deep to see the spectacle, many of them with mouths open and eyes wide as they get a close-up view of something that seems impossible.

It's even amazing from far away.

"We can see the freestyle course from the top of the pipe and it's unbelievable and inspiring to see what they're doing," said Gretchen Bleiler, this year's superpipe gold medalist. "It makes what we're doing seem easy."

Freestyle snowmobiling has been around for years, created by racers and backcountry riders who were looking for that little extra thrill. They started by jumping houses and building ramps over gaps — anything they could find.

The first tricks were similar to freestyle motorcycle moves: hanging off the handlebars and seat, landing with no hands, twisting their sleds in the air. Not to be outdone by the moto guys, the slednecks started flipping their rides a little over a year ago, creating a spectacle that Evel Knievel might have backed away from.

"This is absolutely amazing," said fan Brian Peterson, who drove up from Denver with some friends to see the flipping snowmobiles firsthand. "You watch it and it's almost like you can't believe what you're seeing."

Perhaps the most amazing part of flipping a snowmobile is how easy the riders make it look.

To get something that big to flip over backward would seem to need an incredible amount of exertion. But the riders look almost nonchalant as they rotate around, using a short burst of power and a tug of the handlebars get started.

Some rotate so slowly, you wonder if they're not going land upside down, pancaked by a 500-pound machine. It hasn't happened yet, though the thought is constantly on the riders' minds.

"To take a snowmobile and put it in motion to go upside down, that still scares me, and I've done it quite a bit," Burandt said.

The hard part is getting up the nerve to try it for the first time.

Just ask Burandt.

He started riding snowmobiles when he was 5 and became an accomplished backcountry rider and racer. But when Burandt came into the inaugural Winter X freestyle competition knowing he would have to pull off a backflip to have any chance of winning, he had trouble making himself do it, needing more than two dozen attempts to finally get up the nerve.

"I physically couldn't get myself to do it," said Burandt, who took gold last year. "Every time I would come around the berm and look at the ramp, I would just go into shutdown mode. I was getting so (angry) at myself because it was two weeks before X Games and I had to do it or there would be no chance of winning.

"To land that first trick to snow is a feeling I'll never forget, and I get to take that one with me to the grave."