This can only be done in one specific location. Approximately halfway through the game, you will reach a Coloseum, where you will get attacked by four enemies. After that, Trico must open a gate which reveals a cart. To progress in the game, you must push the cart to the middle of the arena, then let Trico propell you upwards where you can hold on to some metal. However, you do not want to hold on to the metal at the ceiling to get the "Broad-Backed" trophy. Place the cart close to the middle of the room, just outside the lighted area. Wait for Trico to step on the cart, and you should automatically land on his back. This is the only cart in the game and a very memorable location. You will know it when you see it. The cart is unmissable because you need it to progress in the story.