People got way too obsessed with “perfect form” on curls. If the weight is so light that your torso doesn’t even need to stabilize, it’s probably not heavy enough to force growth. Look, don't spend all your time doing biceps curls trying not to move the shoulders. GET THEM HEAVY. See arnold:
Heavy standing curls where you cheat the concentric a bit, then fight the eccentric under control, are brutally effective. 5 hard reps with a weight that actually scares you will build more real-world arm strength and thickness than endless pump sets. Your biceps aren’t isolated in real life anyway. Heavy curls turn the movement into a full-body effort, grip, forearms, shoulders, core, balance, all working together to move heavy shit. It's more fun AND it turns out it works better.
The key is controlling the negative. Don’t just throw it down. Make the eccentric miserablely slow (4-5s). That’s where a lot of the growth happens. Stop focusing so much on technique, just get heavy shit up and put it back down.