I am reading 'Mastering Bitcoin' to study for bitcoin and blockchain. At page 295-296, Author described that two parties should exchange their revocation key before signing new commitment transaction to punish when either party tries to cheat.
But, if we use this method, either party can redeem prior commitment transaction by using revocation key whenever it wants. This mechanism is being used by lightning network now. I wonder how this method prevents either party from cheating.