The proper execution of the repetition is the single most important physical element in productive training. Intensity and progression will not yield maximum results unless they are performed with perfect form. Swinging around heavy weights will not produce results. Your muscles must control the weight without excessive momentum. Few people use good form.
Few people go all the way down without swinging when curling.
Good form, once you are past the beginner stage of training and are potty trained, should be automatic. Doing a set to muscular failure does not give you the right to get sloppy with your form. You go to failure in perfect form. Any reps done in a sloppy fashion do not count. If one of my clients does a set of 20- rep squats and 5 of them were not to the maximum depth that is safe for that person, he only gets credit for 15 reps.