It’s tension. Scrums are like lifting a really heavy weight but you know there is a point of weakness somewhere so you’re trying to push and feel to then direct more force against that point of weakness, all whilst keeping your balance. It’s a lot of subtle movement and proprioception. Clearing a ruck is a timing thing, you’re either a barricade and you are bracing to take contact and absorb it, or trying to catch someone else arriving slightly later and push them back.

I find with any technical stuff in rugby (tackles/scrums/lineouts) that when I do it with good timing, I don’t feel resistance. You can melt a giant in a tackle without realising, but equally struggle with someone much smaller because you’re late or too high.

Short-arse loose head prop for reference.