Keep people off-balance and in dark by never revealing the purpose behind your action.
If they have no clue what you are up to , they cannot prepare a defense. Guide them enough far down wrong path , envelop them in a enough smoke, and by the time they realize your intention , it will be too late.
Do not reveal what you have thought upon doing , but by wise council keep it secret being determined to carry it into execution.
Resource : The 48 laws of power.