For optimal gains, you need to perform each exercise according to the prescribed tempo code — the first digit refers to how
long (in seconds) it should take you to perform the
eccentric part of the rep (lower the weight), the second digit indicates how
long you should pause at the bottom of the movement, the third how
long it should take you to perform the concentric part of the rep (lift the weight) and the fourth tells you how
long you should pause at the top
portion of the movement.
That being said, make sure to keep the
eccentric portion of the curl at least one second
long in duration to maximize tension and muscle fiber recruitment.