The Proof of Concept Tech is where the developers show that any of the special features for the game are doable in the game engine. For example, in a game where the player can teleport around the level, the Proof of Concept Tech would be a sparcly populated area where the player could teleport around. This milestone proves that the game is possible. If the team is unable to create a Proof of Concept Tech, they will usually go back and change their design to something that can be done.
Previous Milestone: Documentation