How to Perform the Preacher Curl
The bicep brachii comprises roughly one-third of the upper arm and is comprised of the long and short heads. The brachialis, also referred to as the side arm or lower bicep, is commonly considered part of the bicep muscle group even though it's technically different.
The biceps are primarily built through pulling movements such as chin-ups and elbow flexion exercises such as curls. Most bicep movements are performed in vertical plane although the torso angle may also be slightly backward (e.g. incline dumbbell curls) or forward (e.g. concentration curl) depending on the exercise.
Whether you're a top-tier athlete or weekend warrior a beastly set of biceps will set your physique apart from the rest.
The preacher curl is an isolation pull exercise targeting the brachialis, commonly referred to as the lower bicep. The biceps brachii (short & long heads) and brachioradialis (upper-outer forearm) act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group in completing the movement. The wrist flexors as stabilizers during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
The preacher curl is one of the best exercises to build the bicep peak.
How to Perform the Preacher CurlYou can perform the preacher curl using a barbell, EZ bar, cable, or dumbbell. Although this walk-through will focus on the performing the exercise with a barbell, you can apply the general form and tips to all setup variations.
Place a short straight barbell on the pins of the preacher bench and select the appropriate working weight. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions. This working weight will likely be less than the amount you use for traditional barbell curls.
don't place 10lbs on one side and 25lbs on the other. Uneven loading won't improve your gains and will likely lead to an injury. As a reference point, the shorter barbell usually weighs about 30lbs.
Once you've selected the appropriate working to adjust the padded seat so that when you're seated and your arms are straight, but not hyperextended, the armpits rest resting near the top of the triangular inclined pad and your rear upper arms and elbows are completely resting on the inclined pad. While seated lean slightly forward to grasp the barbell on the pins with a shoulder width supinated grip (palms facing you).
You can use a traditional grip (thumb wrapped around the fingers), hook grip (fingers wrapped around the thumb), or a false grip (thumb and fingers on the same side of the bar). Your shoulders should be down and away from your ears. This is your starting position.
Take a deep breath, brace your abdominals, squeeze the barbell as hard as possible and begin pulling your hands towards your shoulders. During this arc motion, your elbows and upper arms should remain in a fixed location and completely in-contact with the inclined pad. Do not allow the upper arms or elbows to drift out of place; doing so will take the stress off the target muscle groups.
Continue curling the barbell until your forearms are close to or perpendicular with the ground. The precise top position will vary with the individual, depending largely on your upper arm strength, form, and shoulder flexibility. While some trainees prefer to curl the barbell until the forearm is perpendicular with the ground, this may require your elbow to drift forward or shoulder to roll-in or raise.
Curl the weight towards your shoulders as much as possible, stopping just before your elbows and shoulders change positions. This cue will maximize engagement of the target muscle groups. Continue squeezing the barbell and flexing the biceps, holding at the top position for 1 to 5 seconds.
Once you've held the barbell at the top of the movement for the desired duration lower it in a slow and controlled motion back to the starting position. The movement pattern for the lowering portion should be the exact reverse of the pulling portion. Complete for the desired number of repetitions.
Some lifters choose to exhale while curling the barbell, at the top of each rep, or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression.
Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc) but strive to improve every training session.
Preacher Curl Form TipsAvoid Momentum - The preacher curl provides maximum benefits when it is performed in a controlled full-range of motion. Check your ego at the door and don't immediately attempt working weights used for traditional barbell curls.
Do not use momentum, raise your hips off the pad, or allow your upper arm and elbow to come off the padded to bounce the barbell from the starting position to the top position. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
Hold the Squeeze - Increase intensity by holding the top position of the preacher curl for 5 to 10 seconds. Really focus on squeezing the biceps and gripping the dumbbell as tightly as possible.
This will increase time under tension and the burn in the biceps. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
References1) Griffing, James, et al. "Barbell Preacher Curl." ExRx.net. N.p., 2015.
2) Griffing, James, et al. "Kinesiology Glossary." ExRx.net. N.p., 2015.