1. Fill your inventory till there is only 3 spots open. There can be NOTHING in your inventory taking up space that drops from the racing kit.
2. Have 1 rumbling sapling in 1 of the 3 open slots
3. Have a racing kit in 1 of the 3 open slots
4. The racing kit always drops 2 items. The exploit is that the game does not let you open the chest if there isnt room for two items to drop into your inventory, it simply says inventory full. It will success open the craft if the rumbling sap and 1 other item drop into your inventory. Remove the random drop to open another crate for more saplings.