Fig 1. Voiding Faces

Fig 1 shows you what you get once you have hit the <V> key; a menu of further key commands down the bottom of the screen.

<LMB> - Voiding Inside a Box 

If you draw out a box with <V><LMB>, then all faces inside this box will be removed.

<RMB> - Voiding Outside a Box 

If, however, you draw out a box with <V><RMB>, then all faces outside the box are deleted.

<G> - Voiding Marked Faces 

You can delete all the green marked faces by typing <V><G>.

<U> - Undo Last Void 

Type <V><U> to undo the last void action. There is only one level of undo though, so you can only undo the last void action, not any previous to that.

<S> - Swapping Voided Faces 

Type <V><S> to swap the deleted and remaining faces; this can be used to recover faces voided before the last void action. Type <V><S>, then void the faces you want to restore, then type <V><S> a second time to get your mesh back.