Eggertsson and Woodford (2003), among others, go as far as suggesting that a promise by the central bank to maintain a zero interest rate policy for some time in the future, and even in the face of conditions that would otherwise dictate a firming of the stance, would be a powerful instrument to stimulate the economy already at present.
In this line of thinking, forward guidance is a communication campaign by which a central bank advertises its intention to depart from its usual strategy, if only temporarily.