The muscles that you use to hold the bow back at full draw and keep it from creeping forward are the same muscles you use to squeeze the trigger off.This was the explanation that made it click in my mind.
Back tension is hard to learn and you need a true back tension to understand the feel.They really aren't that expensive ( some are

)and are a great teaching tool.
www.lancasterarchery.com I have the T.R.U. BALL TRU TENSION ULTRA 3 FINGER and it works great.It is $49.95 and that really isn't that bad.Especially for what it can teach you.