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.
If a trainer were to come to us and say he had a method of ab training that would reduce the waist size of anyone in three weeks, we'd ask him what he was selling and then kick him out the door. Luckily, Don Alessi was a fast talker and before we could introduce him to the sidewalk, he'd convinced us that he had some pretty nifty ideas.