You have to not look at it like a polygonal model. You can select faces and edges but they are only kind of a visual representation of the boundaries of the surface.

Instead think of it as a solid block of metal or wood. If you had a cube and you wanted to taper it on one end you would either shave off some material or add some on the other end. Either way you are changing the volume of the material not pushing it around like clay to get your shape(polygonal).