✅On the left: back stays tight, shoulderblades tuck back together and the chest sticks out.

By keeping the shoulders back, no nerves/tendons passing through your chest get pinched, making you lift the weight safely.
Shoulder is going anti-clockwork wise, so does the arm ✅.

NOTE: some people might say that your elbows should stay attached to your sides, but I believe that's not a natural pattern.

Letting them go a little forward is fine (on the last part of the eccentric) you're still completely isolating your biceps and that's still pretty safe. ✅

❌On the right the dude ego lifts the weight using momentum. Because the weight is too heavy he can't lift his arms without "losing" his shoulder (it basically cannot stay tucked back), so it moves forward causing the nerves & tendons to get pinched.
This happens because his shoulder goes down, while his arm wants to go up.
Different motions cause the tendons in the middle get torched and pinched ❌.

Because the weight is too heavy, the dude goes off the vertical axis aswell, putting himself a little more forward and causing more of the weight to be placed on his lower back.
Boom, back & shoulde pain. ❌