As the eggplant cooks the meaty flesh inside will begin to give up its moisture. Eggplants may seem dry but they hold a lot of water. It is this water that contains the bitter flavor of the eggplant. By roasting the eggplants as hot as you can you will let this liquid drain out through the holes we punched in the sides.
Because they are dense, eggplants can hold a lot of heat and will continue cooking after they are removed from the heat so we don't need them completely heated through, just mostly. Once the skin is crispy and burnt (and loose) remove the eggplants from the heat.