By making the witches say things that have no apparent meaning at the time they were spoken, but which turn out to be very accurate descriptions of what happens as the play unfolds.
You could take any one of their three "predictions " near the end and wonder how that would unfold. I think the easiest to understand is the one about Birnam wood in Act IV
Macbeth shall never vanquished be until / Great Birnam Wood to high Dunsinane Hill / Shall come against him
It seems impossible, but what has to happen is that the trees of Birnam wood must come to Dunsinane Hill (where Macbeth is). The audience (if they have never seen Macbeth before) know that trees don't move, so how can this be?
It happens because the soldiers coming from Birnam wood cut the trees down and are using them as shields. If you are watching this, you have to wonder what is going to make it happen.
