there is one new type of turn you can take, which is to not move any piece, but pick a piece on the board which is yours, and "reinforce" it. a piece which is reinforced can be captured only if the enemy has two ways of capturing it (of course when the capture happens, only one of those ways need be used). once the pieces moves, the reinforcement effect also goes away.
furthermore, you can double reinforce a piece using 2 turns, triple reinforce a piece using 3 turns, etc.