This is a heavily debated thing amongst Christians. I heard many opinions/interpretations for years and personally - I do not know the answer. It is likey that no one does, and we may never know until we get to heaven.
1 Peter 3:18-20 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. KJV
The above scripture I have heard it used to support this idea.
Also, many say that this was when he actively took the keys of death and hell.