The chest control was prepared for translating the upper back in any direction, but it was not yet capable of creating a twist in the back. In order to do this, I used the connection editor to create a connection between one of the attributes of the chest control and the twist attribute of the IK Spline Handle that I created early on.
I loaded the chest control as the output and the IK spline handle as the input. In order to create the connection, I selected Rotate Y from the chest control list, as this was the most appropriate attribute for influencing the twist, as the twist of the spine moves on the Y axis. I selected twist from the list of attributes of the IK spline handle.