The best explanation of releasing with back tension I've heard is not 'squeezing the back muscles together.' Seems a lot of people have a hard time grasping what that means. I like to say 'use your shoulder to push your elbow straight back.'

You can get the feel for that just using your imaginary bow. Hold your bow arm up like you're shooting and your string hand on your normal anchor, just like you're at full draw. Then, just push that string hand elbow straight back. It'll only move an inch or so, but you'll feel the muscles that are involved. If you can't feel 'em, then push it back and push hard when you get it as far as it will go.

Once you feel those muscles, then you'll know what back tension is.

To release a wrist strap style caliper release with back tension, take up whatever slack is in the trigger by squeezing your finger and stop just before the trigger trips. Then, continuing to aim (very important, you know) turn that finger into a rigid hook and push your elbow back. The movement of the elbow pulls your finger thru the trigger to finish the shot.