What is Flex Control?

Game Master Toolbox

I've seen a few products that mention a dial called flex, and I'm hoping it controls the contraction and stretching of the muscle but I want to confirm before I purchase.

 

I've seen this product has it - https://www.daz3d.com/musculature-hd-morphs-bundle-for-genesis-8 https://www.daz3d.com/musculature-hd-morphs-bundle-for-genesis-8

 

I see this product has it as well - Auto Muscle Enhancer HD For Genesis 8 Male(s) https://www.daz3d.com/auto-muscle-enhancer-hd-for-genesis-8-males

 

I have this one for both male and female, and I don't think they flex. https://www.daz3d.com/muscularity-morphs-for-genesis-8-males

I noticed that the triceps and deltoids are a little weird in certain poses. The triceps always looks like the arm is straight, so it always looks contracted. The deltoids don't look stretched when putting them in flying pose - arms out forward like Superman.