You have to marry the rifle to the shoulder, it has to have firm contact, first
Then once you have it in place, you relax behind the rifle, the pressure to the rear does not change, it just maintains, but then you relax the rest of the body so you are a slab of dead meat behind the rifle.
The shoulder and the bicep are two separate muscle groups, the tension in the shoulder is not the rearward pressure of the arm
You relax the shoulders, which is not at cross purposes, two separate things
Clearly a large group accomplishes this so it's not really hard, it's just different ways to say the same thing so people can understand it
The biggest mistake is people holding tension in the shoulders, you bring the rifle to the shoulder first, setting that up as move into position, the bipod holds it in place so you can relax the shoulders at the same time.