This has been one of my favourite French expressions ever since August of last year. I had only recently learned the word caisse as a slang word for "car" and was driving around the Scottish Hebrides with a bunch of French friends. We needed to get the ferry from the Isle of Mull back to the mainland and hadn't realised that we needed to book tickets in advance. Having queued for a good half hour in the lane for last-minute places, we realised that we were unlikely to get a crossing back that evening and decided to go to another port and catch a different boat instead. Unfortunately, the queue had built up and there were about ten cars and a lorry behind us, so we had to enlist the help of Caledonian MacBrayne's delightful staff. The attendant, who was definitely local and must have been pushing sixty, cleared away the traffic cones and signaled to the vehicles in the adjacent lane to move out of our way. All these preparations having taken place, he beckoned with his arm and, to my astonishment, cried, "A fond la caisse!" through the open window of the car.
The expression translates as something like "put your foot down," or "full throttle". As I was driving my mother's car, I didn't quite take things that far, but it was certainly the longest and fastest reverse maneouvre I have ever done in my life.
I was reminded of the incident while we were skiing by a friend who liked to talk about flying down the piste at top speed and was even more delighted when I learned that the word caisse actually refers to a caisse à savon, literally a "soap box" but also a go-kart. Because we all know how much control you have in one of these when you go at full speed!