This flexing ab section thing is not very hard to do and Finhead hit the nail on the head. All it's made of is elastic and plates of plastic. Those arms you see on the side, those are not necessary to the function of this part. They are only for show.
Let's start with the ab template. The main ab plate can be whatever shape you want, so feel free to change it up, play with the templates, etc. The arms on the template might not be the size you need. I made them longer in case someone had a larger midsection and needed the extra length to make it look right. I'll show you how to put it together and find out what's right for you.
I bought 4 yards of 2" wide white elastic.
I wrapped a strip around my chest, where I wanted the top of the first Ab plate to sit. I cut the elastic to size and put a safety pin in to hold the two ends together.
Then I riveted the top of the main ab plate body to the elastic.
You will need to use washers on the back of these rivets. The elastic won't hold if you don't use a washer.
You should use washers on the back all of your rivets, but if you get in the habit of not using washers this is not an area you can do with out them.
At this point if you do this 3 more times, you'll have a nice abdominal chest piece. So, if you are doing War Machine you wouldn't need to add the little arms that come off the main plates. Since we're doing the Mark V, we'll add the arms.
I drilled a hole for the arms to attach to the main plate, which you can see in the above picture.
I cut a quick strip of plastic out and riveted it to the main plate, then attached another strip of plastic to the first one to form an obtuse angle. Some what of an exaggerated "L" shape. The top of the "L" is attached by rivet to the top of the same strip of elastic that the main plate attaches to.
Somehow I missed taking a picture of this, but you see what I mean in the pic below. I'll post a better picture later.
So, like I said, do this 4 times or more depending on how many plates you want to do. If you decide to make the plates smaller you might need more.
When I made this, it was before I uploaded the most recent Ab templates and the arms are too thick. I made them thinner in the templates that are available now. I also had them pined on the elastic to test it out.
At this point you should have something like this:
I just threw this together very quickly to give you an idea of the articulation.
If you want to hide the rivets, you can cut out the fins on the template and glue them over the rivets.
I decided not to hide the rivets, so I skipped this part.
My advice is to lay it out how you plan on assembling it. Hold it to your stomach and see about how long the arms should be for you. Some will need longer arms than others. This is one of those parts where the template won't work for everyone. Luckily it's pretty simple.
Any questions, let me know.