The use of dumbbells provides opportunities for greater variety in doing exercises for biceps. They exert additional effect on the biceps and the arm muscle, as isolation of the muscles is achieved. Here the balancing effect of the barbell is absent and balancing the weights puts specific load on the muscles, contributing to their growth and development. The movement with dumbbells can be performed with undergrip, overgrip and a combination of inward- outward rotation. They can be performed alternating the arms and with the two arms simultaneously.

Alternating or simultaneously working of the arms
We recommend alternating the arms for maximum isolation and stimulation of the respective muscles. Since we are using dumbbells, it is reasonable not to lift them the way we lift the barbell, but use another method. Thus a different effect should be achieved. Using both arms simultaneously does not have the isolating effect on the muscles. One may ask why we should use dumbbells when the same can be achieved with a barbell. The dumbbells allow using bigger weight so a greater number of motive units are activated.

Different grips and planes of movement are possible
With dumbbells we can use a different angle and different grips, so as to shift the accent on the respective muscles. Take as an example the seated variants with the outward grip, when the dumbbells are moved on either side of the body at a 45 degree angle, not in front of it with both arms simultaneously.
Active supination (the corkscrew effect)
Rotating the dumbbells outwards (active supination) requires paying special attention. In it the dumbbells are positioned on either side of the body, half- rotated inwards and then they are half- rotated outwards while lifting them. During the eccentric phase the process is reverse. The dumbbells are rotated inwards while lowering them to the initial position. The inward and outward rotation takes place in the middle of the movement.
We must know that the contraction of the biceps takes place not only during the flexing of the elbow joint, but while rotating the forearm outwards as well. Supination exerts an effect of twisting on the biceps and stimulates strongly the long and short heads. It is believed that its optimum development cannot be achieved without applying supination. A lot of famous bodybuilders claim that all the curling exercises in the world cannot contribute as much to the development of the biceps as supination.

An easy-to-perform experiment
An interesting way to understand the importance of supination is to place one hand slightly bent with inclined overgrip. You don’t need to use weight. Slowly rotate the palm upwards. You will feel a tangible tingling and tightening of the biceps. This can hardly be felt when using additional weight. 
The term “active supination”
Applying supination during the movement is different from rotating the arms before and during the two phases of the movement. When the arms are turned outwards, the long and short heads of the muscles are activated all the time, which gives opportunity to hold this outward position. The opponents of the “corkscrew” point out that it does not allow activation of the short head at the final part of the eccentric phase as the forearm is extended downwards. The outward rotation removes the accent on the muscles and activates the arm and the arm-elbow muscles.
The countercharge in favour of active supination is that the beforehand outward position of the wrist does not allow twisting while lifting the weight. The possibility to apply the “corkscrew” effect is a better compensation for the outward rotation of the wrist at the end of the eccentric phase. It would be useful to include active supination in the movement.