We need to engage the deltoids, pectoral muscles and trapezium to throw the arm away from the body.
A great way of understanding the feel of this either try or imagine using a chest expander. You force the chest expander apart using the large core muscles. If you try and open the chest expander using the arms, the exercise is almost impossible and you can feel the stress on the arms.
When you have the feeling of the chest expander, then swing the racket using the same muscles and with the same feeling..
You will immediately feel the strength and stability of the stroke.
Use the core secret of the one handed backhand and turn your backhand into a weapon.