You would never see it. Just like there are multiple carousel in bag claim, there are multiple in back. Only difference is the ones in back are where the bag goes to be put on a cart instead of where the bag goes to be picked up by passenger after unloading from cart.

Example -

Checkin bag -> goes thru belts -> ends at bag carousel in back where all bags for flight go -> bag is picked up put on cart or loaded in container -> put on plane

Plane arrives -> bags put on cart/container unloaded -> bags put on belts -> either

1)for destination passengers, bags feed to bag carousel in bag claim for their flight or
2)connecting passengers bags sent to carousel in bag handling in back designated for their next flight -> bag is picked up and put on cart or in container for the flight -> bag/container loaded -> repeat Plane arrives untill passenger final destination

If there is no cart/container ready for the flight then bag goes round and round in back till one is ready. At this point the bag is picked up and put in cart/container.