What are the dead in hell waiting for before being cast into the lake of fire?
According to the context, the dead in hell are waiting for the great white throne judgment and their resurrection out of hell. They are delivered up so they can be judged, every man according to his works, and then, having been occupying hell, they will be cast into the lake of fire, which is the second death.
Les explains that death and hell is the place of the unbeliever who has died, and they are waiting for the great white throne judgment. Hell will give up those in it so they can be judged, every man according to their works. After that judgment, those who had been occupying hell are cast into the lake of fire. Les notes that this is something far worse than hell and comprises the second death—physical death, and now the spiritual. He also defines death as separation: physically the spirit from the body, and in the spirit realm separation from God for eternal separation. He contrasts hell and the lake of fire, pointing out the difference between the two places. The sea gives up the dead in it, and death and hell deliver up the dead in them for this judgment.