“Tombeau ouvert” literally means “open tomb”, but when you say someone is moving/running/riding/driving “à tombeau ouvert”, it means they are going hell bent for leather, at breakneck speed, etc.
On July 5th, 1964, a rest day in Andorra at the Tour de France, Jacques Anquetil got into a sangria drinking contest with his directeur sportif, Raphaël Géminiani (amazingly, still alive and living in Clermont Ferrand). The legend goes that a famous astrologer foresaw that Anquetil was going to get killed on the road of this Tour de France, and Jacques was living like there was no tomorrow.
The next day (Andorra to Toulouse), after catching wind of Jacques’ bender the day before, Raymond Poulidor and Federico Bahamontes attacked from the gun, distancing him on the climb to the Col d’Envalira (incidentally, the highest point of the 2021 Tour – 2408m).
At the summit, Anquetil downed a water bottle full of dry champagne and proceeded to descend à tombeau ouvert into heavy fog, guessing where the road was from the lights of the race cars in front of him. He caught the breakaway and eventually went on to win his 5th and final Tour de France.
Apart from the language lesson, I’m sure there’s a moral in there involving champagne, but I’ll leave that up to you to figure out.