An employee was working on the slider section of a flatbed trailer tandem to determine why the slide lock mechanism was not functioning properly. Repairs were made and the employee needed the trailer tested to ensure proper function of the lock. During the testing process, the trailer brakes are applied and a pallet jack is used to pull the trailer, allowing the lock to engage. His middle finger entered the pin slot right as the trailer was starting to move. When this happened, the plate caught his middle fingertip, causing a deep laceration that resulted in a partial, surgical fingertip amputation.