Teshuvat Hamishkal is a mystical practice prescribing repentance to make amends for a particular sin.
It was prescribed by kabbalists, especially in cases where the usual methods of repentance (confession, resolve for the future, asking forgiveness of the person we have wronged, offering monetary restitution) seemed unable to fully remedy the damage done by the original action. The mystics would perceive what needed to be fixed and what spiritual practices could effect the necessary transformation.
Gossip and slander are classic examples of sins where it is virtually impossible to undo the damage done. A lie will travel half way round the world before the truth can even get its boots on. Rav Kook gave as an example of Teshuvat Hamishkal our ongoing effort to make good the sin of the spies sent by Moses. The spies slandered the Land of Israel, demoralising the people and causing 40 years of wondering. Our task in remedying this sin, Rav Kook writes, is to declare to the entire world the land’s “magnificence and beauty, its holiness and grandeur”.