Thomas Aquinas, Compendium of Theology

On the part of the soul, death among men is followed, in consequence of sin, by descent into hell, not only as a place, but as a state of punishment. However, just as Christ's body was buried beneath the earth regarded as a place but not with respect to the common defect of dissolution, so His soul went down to hell as a place, not to undergo punishment there, but rather to release from punishment others who were detained there because of the sin of the first parent for which He had already made full satisfaction by suffering death. Hence nothing remained to be suffered after death, and so without undergoing any punishment He descended locally into hell that He might manifest Himself as the Savior of the living and the dead. For this reason He alone among the dead is said to have been free, since His soul was not subject to punishment in hell and His body was not subject to corruption in the grave.

When Christ descended into hell He freed those who were detained there for the sin of our first parent, but left behind those who were being punished for their own sins. And so He is said to have bitten into hell but not to have swallowed it, for He freed a part and left a part.

The Creed of our faith touches on the various defects of Christ when it states: "He suffered under Pontius Pilate, was crucified, died and was buried; He descended into hell."