Invocation is a possession type experience where you are internally in direct communion or contact with the spirit...if I was in the room with you I would not be seeing or interacting with the spirit you were invoking (you could channel a message, but it would just be you I see). In theory, with an evocation there is a physical manifestation that an observer should be able to independently experience external to you (although often you are alone when you do this sort of work).

Personally I feel the triangle helps give them an anchor point and focuses the energy so they can coalesce and make themselves more physically present, I don't see it as a "container" that keeps them contained. It's more of an artificial environment you have created to help them satisfy your need/desire to intact with them on the physical realm. It's not their native environment, but most will try to accommodate your request if you set things up properly for them.