How to Perform the Decline Bench Press
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.
The barbell decline bench press is a compound horizontal push exercise targeting sternal head of the pectoralis major. The clavicular head of the pectoralis major, anterior or front deltoid, and triceps brachii (comprised of the long, lateral, and medial heads) act as supporting muscle groups during this movement. 
Supporting muscle groups assist the target muscle group(s) during the movement. The short head of the biceps brachii act as stabilizers during this exercise.  Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving. 
Contrary to popular belief the barbell decline bench press does not isolate the lower chest; it includes both major heads of the pectoralis major.
How to Perform the Barbell Decline Bench PressApproach the decline barbell bench apparatus and select the appropriate working weight. Ensure you add an even amount of weight to both sides of the bar. don't place 90lbs on one sides and 125lbs on the other side. Uneven loading won't improve your gains and will likely lead to an injury.
If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions. Most trainees find that when the number of reps is held consistent, they're able to use more weight on the decline bench press compared to the flat bench press.
Once you've selected the appropriate working weight place your feet and ankles underneath the padded leg brace and then lay backwards so that you're lying face-up, with your head, upper back, and glutes completely touching the padded support. Keeping your chest high, shoulders down, and squeezing your shoulder blades and upper back together, grasp the bar with a traditional pronated grip (thumbs wrapped around the knuckles and palms facing away from you) wider than shoulder width.
There is no universally recommended grip width for the decline barbell bench press; it will vary based on limb length and shoulder mobility. Aim to use a grip in which your forearms are perpendicular with the ground at the bottom of the lift, when the bar is touching or nearly touching your chest. An excessively narrow grip moves the emphasis towards the triceps and an excessively wide grip decreases the range of motion and increases the likelihood of shoulder impingements and strains.
After setting your grip take a deep breath, brace your abdominals for impact, and with the assistance of a spotter or by yourself lift the barbell off the pins. Position the barbell so that it's vertically in-line with the lower portion of your pecs.
Lower the barbell towards your chest in a controlled motion. As you lower the barbell ensure your chest stays up, shoulders stay retracted, and upper back remains squeezed. Once the barbell hits the required depth, which for most will be touching or slightly above the lower portion of the pecs, hold for 0 to 5 seconds.
After holding in the bottom position for the desired duration press the bar upwards until the arms and elbows are straight (but not hyperextended). The movement pattern for both the lowering and pressing of the bar should be the same. Some lifters choose to exhale while pressing the barbell 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.
Decline Bench Press Form TipsHold at the Bottom - If you're looking to increase intensity then experiment with holding the barbell at the bottom position, touching or slightly above your chest, for 5 to 10 seconds. don't relax your muscles and simply let the barbell rest on your body; this is a receipt for serious injury.
This hold will not only stretch your chest but will also increase time under tension. Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Stay Tight - Staying tight throughout the entire movement minimizes the likelihood of injury. If you've ever seen someone lose tightness during the lowering or pressing portion of the bench press, then you know just how severe the consequences can be. Staying tight begins with a proper setup prior to removing the bar from the pins.
Keep your chest up, shoulders down, and squeeze your shoulder blades together as hard as possible. Those cues, combined with taking a deep breath and bracing your abdominals for impact prior to unracking the bar, will ensure your shoulders and pecs stay healthy for a longer period of time.
Five Points of Contact - Throughout the entire lift your ankles should be touching and secured underneath the padded leg brace and your hamstrings, head, glutes, and upper back should be touching the padded support. Keeping all five points of contact will help ensure you stay tight and encourage the completion of high quality repetitions.
Avoid Momentum - The decline barbell bench press 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 225lbs if your 1-rep maximum on the flat barbell bench press is only 135lbs.
In addition to staying tight and keeping your head, glutes, and upper back on the padded support, do not bounce the bar off of your chest. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
Avoid the False Grip - Do not use the false grip (thumbs and fingers wrapped around the same side of the bar) unless you have a preexisting injury preventing you from using the traditional grip (thumbs wrapped over the knuckles).
While many trainees find they're able to press slightly more weight using the false grip, the likelihood of the bar rolling out of your palms and on to your chest or neck exponentially increases, especially when laying at a decline angle. If you insist on using this unsecure grip ensure you have a spotter or safety pins in place in case you lose control of the bar.
References1) "Barbell Decline Bench Press." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 3 Nov. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 3 Nov. 2015.