This is a simple exercise that helps isolate the biceps, particularly the long head. Anytime you are performing an isolation exercise is easy to take tension off the muscle because the weight you are using is light in the position that you have to perform the exercise and his exact. The concentration curl is no exception to this. As mentioned in the video there are many different ways to perform this exercise.
Seated is the simplest way and my personal favorite due to the fact you can concentrate on the movement more. Like most isolation movements this is a light weight exercise. It is more about working the muscle rather than trying to go up in weight as often as possible. Biceps are small and relatively weak muscles compared to most of the other things people would concentrate on in the gym so you won’t be impressing anyone by curling a lot of weight. Concentrate on the range of motion and working the muscle you are focusing on.