Details are sketchy at this time, but it sounds like Jung von Matt/Neckar ran an internal competition at their agency to see what ideas employees could come up with that would get big attention — whether current clients or not. One team came up with an idea to get competitors' deliverymen to promote DHL. Boxes were coated with a temperature sensitive paint that originally appeared black but upon warming up during delivery would reveal the message, "DHL is Faster." Fuckin genius.
While the agency did warn DHL (who's not a current client) that they'd be executing the concept, the brand didn't know they'd release the video below capturing the stunt. 715,000 and counting YouTube views later, DHL is probably now knocking on the door.