I do not know how much of this is true...

Since I started weight training myself, I let the feeling of my muscles at work be my guide instead of following forms reflected in a mirror.

"By muscle-mind connection I tried to get feedback on how the intended muscles were being targeted".

By doing so, I learned that it is the most natural thing for you to create momentum and adopt "unnatural" positions in order to overcome a resistance that challenges you beyond the comfort zone.

I found that while executing a standing barbell biceps curl, for instance, once I start getting fatigued, my body automatically adjusts and redistributes the resistance by recruiting my back muscles to their maximum and making me lean back. This gives me the apparently awkward appearance of doing it wrong.  But, this is what is really happening... as I hold the barbell in my hands, being pulled forward is the first action that takes place and the arched position is just the result of my body counteracting the front forward resistance.

Nowadays, I believe this is the safest way for me to do the biceps curl, otherwise, I will be setting myself up for an injury.

Without arching and leaning back, my spinal extensor muscles would be fighting the forward resistance in a not fully contracted position (given you are lifting a heavy weight) which could set them up for a spasm.

By arching and leaning back, I create a more solid foundation to flex my elbows and execute the barbell biceps curl. However, if I have to lean back beyond of what a fully extended back would; then, I will be probably using too much weight and putting excessive forces up on my lower spine.