Summary: Performing the bench press maneuver with the bar beginning on pins at chest height increases the workload done by your muscles. In theory this should lead to greater improvement in the size and strength of active muscles. Rest the bar on the pins for 2 or 3 seconds following each repetition to decrease workload on the tendons and maximize benefits to the chest, shoulders, and arms.
Begin with bar resting on pins at chest height
Lift the bar with maximum effort and explosion as you would a normal bench press
Return the bar to pins and rest for 2-3 seconds before repeating
Performing the lift in this manner increases muscular work
This should translate to a stronger bench press and greater muscular development
Added benefit of less strain on joints, particularly the shoulder
The Bench Press is one of the most popular and commonly used exercises for developing upper-body strength and muscle. Chances are, if you’ve ever participated in resistance training, you've performed a bench press.
For those of you who know, I apologize for being redundant, for those who are not familiar: the bench press involves lying supine (back down) on the bench with five points of contact (both feet, buttocks, shoulders, and head) on the floor and bench. The hands are placed on the bar at approximately shoulder width or slightly wider. Arms begin in an extended position toward the ceiling and the bar is lowered toward the chest in a controlled manner before being accelerated upwards by a forcible contraction of the pectoralis major, triceps brachii, and anterior deltoid muscles.
At the Rack, we like to use a variation of this exercise that significantly increases the workload done by the agonizing (main mover) muscles in this lift. By performing the bench press inside of the squat rack with the safety handles placed just above chest height, and beginning the lift with the bar on the handles, avoiding the lowering phase of the movement, we place much greater emphasis on the work done by our active musculature. This form of bench press is typically referred to as the Pin Press or Rack Press.
During the lowering (eccentric) phase of the normal bench press the end range of motion as we approach our chest involves lengthening of the series elastic component in our muscles (specifically tendons). These tissues, as their name indicates, move in an elastic manner, meaning that they are able to be stretched and return themselves to their normal resting length without conscious effort on the part of the nervous system. During a regular bench press the series elastic component is stretched and assists the agonist muscles in changing the direction of the bar from downward to upward. As the bar is lowered, the series elastic components elongate and rapidly contract, helping to properly the bar upwards, along with the aforementioned muscles. 
By beginning the lift on the safety handles in the squat rack with no lowering of the bar prior to the press movement, the series elastic component is not properly stretched and its contributions to the raising of the bar are negated. This is not a bad thing although it does cause the concentric phase (raising the bar) to be significantly more difficult. The reason is that the pectorals major, triceps brachii, and anterior deltoid are responsible for the a greater amount of the work done to raise the bar during the lift. This causes greater neuromuscular activation, more micro tears in the muscle fiber, and a greater hypertrophic response. Thus, although performing the bench press off of the safety handles or pins is significantly harder, it often leads to increased physiological responses. Therefore, when a lifter returns to the normal mode of bench press after including pin press in their training, they should find significantly increased strength gains.
 Greg Wilson et. al. The Effect of Imposing a Delay During Bench Press. Medicine and Science in Sports and Exercise, March 1991.
 Robert Newton et. al. Kinematics, Kinetics, and Muscle Activation During Explosive Upper Body Movements. Journal of Applied Biomechanics, 1996.