This is named after the famous Le Mans 24 hour motor race in France where instead of lining up on a starting grid like Formula One races, the drivers used to have to run to their cars, start the engines and then go. 
At the Conwy Ascent, the boats are lined up halfway down the beach, and the paddlers line up behind a tape at the top of the beach. On the 'GO' the paddlers run to their boats, pick them up and run to the water where they get into the boat and paddle off. As with all races the start is one of the most critical parts of the race, so it may be worth practising this prior to the event!