From TF2 Wiki
Developer Commentary is a mode selectable on the main menu in which you can enter one of 3 maps (Hydro, Gravelpit, or Well) and listen to the developers describing the design process for each map, as well as for the various weapons, features and classes in the game. The player enters the designated spawn room and can travel to various different commentary nodes by pressing the alt-fire button (left trigger on the 360). The player can activate a commentary node by pressing the primary fire button (right trigger on the 360) while standing next to it. Occasionally the game will take control of the camera to show the player a specific angle or section.
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