The setup is simple: one team spawns as Team Fortress 2 props and scurries to hide, while the other all-Pyro team must seek out and identify those props from the rest of the scenery. It all results in some fairly hilarious situations, such as the sequence beginning at 4:25.

More info on the mod, and servers where it's being played, can be found in the Steam forum thread. Thanks to fullbright for the link.