How to Perform the Dumbbell Pullover
The pectoralis major, also known as the chest, pectorals, or pecs, is made up of two heads - the clavicular head, also known as the upper chest or upper pectorals, and the sternal head, also referred to as the chest or lower pectorals. The chest is primarily built through pressing movements and movements requiring the arm to move across the midline of upper body, across the chest.
Related: 4 Chest Workouts for Improved Size and Strength
The dumbbell pullover is an isolation push movement targeting the sternal head of the pectoralis major and involving the movement of the arms vertically across the midline of the chest. The latissimus dorsi, teres major (outer back), long head of the triceps, posterior or rear deltoids, pectoralis minor, rhomboids (middle back), and levator scapulae (rear neck) act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group(s) during the movement. The triceps brachii (comprised of the long, lateral, and medial heads), anterior or front deltoids, clavicular head of the pectoralis major, and wrist flexors act as stabilizers during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
The dumbbell pullover was a staple exercise during the Golden Era of bodybuilding for building the chest.
How to Perform the Dumbbell PulloverApproach the dumbbell rack and select the appropriate working weight. You will only need one dumbbell as this is a single dumbbell exercise. If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions. When in doubt, start with a conservative weight and work your way up; this will leave more room for progression and ensure you develop properly form early on.
Once you've selected the appropriate working weight grasp the dumbbell and place it near a flat bench. You have two options for positioning your body and performing the dumbbell pullover.
One option is to place your upper back and neck on the middle of the flat bench so that your torso is perpendicular to the flat bench. To position your body using this option take a shoulder-width or slightly wider stance.
Engage the glutes, push through you heels, and push your hips up off the ground so that your upper thighs, hips, and upper body are roughly in-line with each other. Some trainees they find a better stretch and chest contraction when the hips are slightly below parallel with the ground.
Your shins should be vertical and perpendicular with the ground. The second option is to place your glutes, back, and upper neck on the flat bench as you would for a traditional flat bench press.
To ensure your arms have enough space to stretch for the pullover movement and the dumbbell doesn't hit the bench, scoot up on bench so that most of your head is resting off the bench. Take a shoulder width or slightly wider stance, ensuring your shins are vertical and perpendicular with the ground.
Place the dumbbell in a position that will be easy to grab. This could be on the floor next to you, behind you, or on the bench. Once you've decided on your dumbbell and body starting positions grab the dumbbell securely.
The most secure way to hold a dumbbell during this exercise is to flip the dumbbell vertically and place both hands together so that your knuckles are facing you and you're supporting the inner portion of higher end of the dumbbell. For those familiar with pop culture this grip may be referred to as the Illuminati hand sign which is also commonly used by the hip-hop rapper Jay-Z.
After grasping the securely grasping the dumbbell straighten your arms, leave a slight bend in your elbows, and position the dumbbell so it's in-line with your lower chest. This will be your starting position.
Take a deep breath, brace your abdominals for impact and begin lowering the dumbbell behind your head. Your arms and elbows should maintain a fixed position throughout this lowering portion. If you find your elbows excessively bending then the weight is too heavy.
Continue lowering the dumbbell until your upper arm is in-line with your torso, hips, and knees. Hold this bottom, stretched position for 1 to 5 seconds. Initiate the return to the starting position by flexing the chest, maintaining a fixed arm and elbow position, and pulling the dumbbell up and back to the starting position. Complete for the desired number of repetitions.
The movement path for lower and raising the weight should be the same. Some lifters choose to exhale while pulling the dumbbell from the bottom position 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.
Dumbbell Pullover Form TipsHold the Stretch - If you're looking to increase intensity then experiment with holding the bottom, stretched position of the dumbbell pullover for 5 to 10 seconds. Really focus on squeezing the pectoral muscles.
This will increase time under tension and the burn in the pectorals. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Avoid Momentum - The dumbbell pullover provides maximum benefits when it's perform in a controlled full-range of motion. Check your ego at the door and don't immediately attempt the 100lb dumbbell. In addition to staying tight (glutes flexed and knees not caving in), keep your upper back on the padded support.
Do not use momentum or raise your hips above parallel to bounce the dumbbell from the bottom position to the starting position. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
References1) "Dumbbell Pullover." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.