When selecting components on a model you often only want to select the components you can see and not the components behind them. You can use Camera based selection to limit your marquee select to only what the camera can see.

To pull out the chin

1. Double-click the Select Tool icon in the toolbox.

   The Tool Settings editor appears.

2. In the Common Selection Options section, underneath Marquee, turn on Camera based selection.
3. -click the head and select Vertex from the marking menu.

   The vertices appear on the mesh.

4. While looking at the front of the head, select the five vertices on the same vertex loop at the front of the chin by holding the left mouse button at one corner of them dragging a marquee selection over them.

   Only the five vertices in the front are selected.

   
5. Press ‘b’ to turn on Soft Select.

   A colored falloff appears on the surface of the face.

6. Hold the ‘b’ key and drag left or right to adjust the Falloff radius so that it encompasses only the chin.
7. Select the Move Tool and drag the chin down and away from the face so that it becomes longer and more angular.
   

Now you have a longer chin without affecting any of the geometry on the neck, which may have occurred accidentally if Camera based selection had been off.