We’ve all seen people in the gym swinging weights around and not following proper form. Let me explain why they won't be making any gains and could possibly hurt themselves.
First of all, the purpose of weight training is to subject the muscle fibers to a work over-load. This process breaks down the muscle fibers and signals the body that it must build the muscle back up in order to lift the weight easily the next time it is attempted.
When you rush though a rep or cheat, you are not giving the muscle the over-load that it needs to break down the fibers. All you are doing is using momentum to move the weight. This does not build muscle.
Each rep should be done slowly and deliberately. You should make the muscle work for every inch that the weight is moved. That is how the muscle grows and strength increases.
As for proper form, this also goes to the matter of cheating and of not working the muscle properly. One other thing it can do is cause injury.
When you swing the weight while doing standing bicep curls, you are putting a tremendous strain on your back. You are just looking for an injury.
Please follow my advice. Take it slow and use proper form. Your gains will come faster this way.