Sam,
You are not alone. Expansion is a pretty detailed step in the shot sequence of the NTS BUT, MAYBE overstressed in many cases by those of us that shoot heavier Trad style bows as opposed to lighter target style bows. If "expansion" or its thought works for you fine but if it is or causes a problem, leave it out.
Ideally (and again referring to the NTS) At "holding" (which happens AFTER "transfer") Your arms and chest muscles will be at their LOWEST tension level -- essentially relaxed with the back holding the bow weight.
IF the muscles in front (chest) are relaxed, you cannot pull (tear up) or injure them. Also, the back MUST take over the bow weight, IF they "slacken" you will be collapsing - most likely.
IMO, the best way to learn expansion is with a VERY light bow or even the rubber band I show in one of my videos. Trying it for the first few times with a heavier bow (> 30 or so pounds) can easily lead to poor execution of the step and injury.
I would go so far as to say, get to "holding" set your aim, and pull through the shot like you would with a form master. Don't stress over the "expansion" step BUT if you MUST, find a coach that can physically and in person guide you through the process.
Trying to execute a misunderstood (or poorly understood) step IS a good way to form a bad habit and even cause injury. I'll say again, "expansion" is a detailed step in a very formal training system but it is just that. A step of explanation. Most that have a good release and follow through are pretty much automatically "expanding" without thinking, stressing, or having it as a conscious step in their shot.
I've suggested this before but maybe it bears repeating here; " Think move the bones, NOT activate specific muscles!" IF you have moved your bones to the right place, you will automatically have the muscle tension you need. Thinking "activate a muscle," is actually very counter productive. MOVE BONES!
Arne