AK.
I have an TruBall Extreme the way I do back tension with it is once I am at full draw I loop my thumb around the trigger so my thumb and index finger are touching see pic.
Then all I do is squeeze my shoulder blades together which in turn applies the pressure to the trigger to get it to fire.
The best way to practice this is to blind bale in other words once you come to full draw and your pin is on target then close your eyes, concentrate on squeezing your shoulder blades together and the fireing should give you a surprise.
I blind bale before each practice session atleast 10 to 15 shots it helps take your mind out of the fireing sequence.