It was run ten times from 1922 until 1934. That was the year the Mercedes-Benz W25 won and set a track record at this 21.5 KM course of Switzerland switchbacks. It decided to come back in September of this year, 77 years later, and run up the hill again. Quite often done with large amounts of right foot and not always straight on exit. It didn't get close to the real flooring it was given back in it's heyday with it's wailing supercharged 3364cc straight-8 on gravel roads with skinny knobby tires. You might remember this car from when Jenson Button gave it a shot at Goodwood 2009.
In 1922 the race up to the Klausen pass was the most difficult and most popular race of the time. It was also on gravel with 136 curves on knobby thin tires! Superstars like Nuvolari, Stuck and Varzi were the measure of what it took. Alfa's, Jags, MG's and others competed. There weren't just cars but also motorcycles, some with sidecars!
Today, since 1993, they try to regularly have a memorial gathering every four years, (Their Swiss timing is a little off on that with the latest run) with entry lists full of amazing machines that would make any regular car show seem lazy. This place has the equipment fired up and raging up the hill with a bit more than touring boot-fulls of fun.
I really think this should be one of the top events to see on anyone's list along with Goodwood's festival, a trip to the Monaco GP, and seeing Reno's air races.