I have this rule when I train clients- which many of them are bittersweet about.
"If it's easy then you're probably not doing it correctly"
This is in respect to their position and mechanics during different exercises.
This isn't meant as a "no pain no gain" type rule- but more of a "you're body likes to cheat so if it feels easy, you probably aren't in the right position".
Our form is our function. Yes, efficiency is something we all desire- including our brain- but efficiency out of laziness (on the nervous system's part) only leads to injury and illness down the road.
I talk a lot about form, motor control, and movement patterning with my clients because, frankly, them knowing how to move and having an internal guideline of the correct way to move means that they will have long-term health success.
What is health success?
That may mean something different to each individual. To me it means having the ability to resist major health issue and injury (baring uncontrollable trauma), and the ability to maintain regular, healthy movement.
To not wake up full of aches and pains, and go to bed feeling the same.
To not worry about falling, throwing your back out, or degenerative osteoarthritis.
To know that if you do catch a cold or flu, your system is primed to recover. And that if you do experience a physical mishap/injury- you know what to do to get back on track.
I strive to create independence in my clients- not dependence. Them knowing that "oh, this feels like it isn't working the right things" feeling means that they will either ask a question to find out, or self- analyze and adjust the exercise appropriately. It means that they are thinking and investing constantly in their health and wellbeing. That means I've done my job.
Our bodies will always move us. By nature, they're always moving in some capacity. Whether they move us correctly depends on our awareness, and to build that awareness is what my purpose as a movement professional is. Certainly, clients remain on as clients because they find value in training with a professional consistently- and I myself hire another trainer to do just that for me- but by teaching a client how to move and how to understand their movement through building their internal awareness- they will be healthier in every aspect of their lives.
As the famous Grey Cook says, "move WELL, and move often".