tacan tells what direction and distance is to the ship. if the tacan and the radial dont line up then you have to fly offset to the tacan until they do. eg say the tacan is 180 degrees (so youre due north of the carrier) and the brc is 270 (carrier heading west). if you wanted to come in directly behind the boat then youd have to travel somewhere south east until the tacan lined up with the radial, then you turn into the boat.
in reality you dont need to arrive at the boat from behind in case 1 landings, you can enter the stack/pattern from any angle