Much like the Scene Capture Cube, a Scene Capture 2D makes an image of the scene on every frame. In fact, Scene Capture 2D Actors do not work like real reflections; they work more like a TV screen receiving an image from a camera. The Scene Capture 2D captures the scene from its view frustum, stores that view as an image, which is then used within a Material. In fact, the Scene Capture 2D representation in Unreal Editor is a camera. As such, they are most commonly used for security camera effects in game.
While only 1/6 as expensive as the Scene Capture Cube, care should still be used with the Scene Capture 2D, as they still require extra processing.