The preacher curl works the lower bicep like no other exercise, due to the elbows being forward, there is always tension on the muscle, especially the lower part.
But the preacher curl should not be performed as you might know it. Machines are useless, so don’t use them. You probably wouldn’t have seen the preacher bench I’m referring to, as it’s not quite common. The sloping side, where you rest your triceps on should be oval shaped. The versions with the flat slope don’t work nearly as well.
When preparing yourself on the preacher stand, put your right foot in front of you and your left foot backwards. Keep your stomach against the bench, with the top of the bench about 3 inches under your lower chest. Incline your shoulders forward. Keep your elbows close to each other and hands a little wider than shoulder-width apart on the bar. Use a false grip (thumbs under and not gripping over the bar). Curl the bar towards your shoulders. Lean slightly forwards at the top position of the exercise so that there is constant tension of the biceps, and lean more backwards when lowering the weight. Never lean backwards when curling up.
Curl the weight in slow smooth movements, not exceeding 2-3 seconds through the movement (top to bottom). Squeeze at the top of the movement, for ultimate contraction. The most important thing is to get a perfect mind to muscle connection going. Picture how you want your bicep to look and keep that picture in your mind when doing this exercise. Focus on where you feel the tension in your biceps and strive for perfect form. Never just do a movement, rather drop the weight a bit, and perform the exercise with perfect form slowly. You’ll feel the difference.
Do about 3-4 sets on this exercise, 6-12 reps and rest no longer than 60 seconds between sets. You can perform this exercise 2-3 times a week for rapid gains.