http://youtu.be/ZawrdYA0WXU
As he says, the principal problem with working the arms, biceps or triceps, is that the movement comes from the elbow joint only. It is worth thinking carefully about this. The arms have 3 moving joints; at the shoulder, the elbow and the wrist. If really big biceps are desired, then the movement has to come solely from the elbow joint ((To really hit the biceps,when doing a palm up curl, then the wrists should be cocked backwards. The resultant weight that can be moved with good form will be ego-challenging)). A cause of swinging the weight or rolling the shoulders forwards to assist lifting the dumbbell comes naturally from the brain wanting to make things easier for the biceps. So it is an exercise worth doing with a trainer or a spotter As the biceps tire, then the desire to swing or roll becomes almost irresistable. To keep the squeeze in the biceps requires great levels of determination and concentration. Incidentally, if form is good, it is also a very good ab workout.
To reiterate what he says, don’t fall into the trap of lifting too heavy a weight. It depends whether big, strong arms are desired more than shifting very heavy weights at whatever cost.