When you curl your arm, the movement is "concentric," which means a shortening under contraction of the target muscle, the biceps.
When you straighten your arm from the curl position, the movement is "eccentric," which is a lengthening of the biceps muscle while contracting.
Some weight training advocates say that specific eccentric training can help build muscle size and strength superior to results from standard concentric-eccentric movements.
A justification for this is that eccentric muscle contraction is stronger than concentric contraction, which allows you to enlist heavier weights. To accomplish this, you may need to employ a helper or spotter to assist with the concentric part of the lift, and then you do the eccentric part alone.
Muscle soreness after a workout may be increased with a greater volume of eccentric work.
The workout program included 3 sets concentric/eccentric plus one set eccentric at increased weight.