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. Think of the standard barbell curl.
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.

This position makes the resistance grow stronger – instead of lessening – as the cable moves downwards toward the body. Hold and squeeze the biceps at the peak contraction for two seconds.
related posts
Cancel that doctor's appointment. Take the self-assessment tests then try these treatments to fix your nagging pain.
If your size and strength gains have stalled out, it's because you made things too complex. It's time to get back to basics and get strong... using only 4 reps.
Here's how to get a thick, wide back... and why what you're doing now is probably holding back your gains.
In this last installment of this series, I want to give you a quick three-week physical peaking routine designed to help you gain some more muscle while increasing muscle density. You should also be able to lose more fat in this stage.