Generally, biceps exercises are set up so the upper arms remain roughly parallel to the torso and the resistance reduces as the hands get closer to the chest. But the guillotine curl requires the upper arms to be positioned perpendicular to the torso with the shoulders flexed (arms forward with respect to the torso). That makes the resistance grow stronger – instead of decreasing – as the cable moves downward toward the body.
This is a great finisher exercise when training biceps. Hold and squeeze the biceps at the peak contraction to further heighten the training effect.