Most muscle units have a three phased strength curve. Weaker, moderate and strongest in simple terms. BP for example. The strongest position is at the top or finish, less strong at the middle range and weakest at the bottom or start position. Push movements ( BP, press, dips,etc) are strongest at the top position and pull movements (cleans, pull-ups, etc) are strongest at the start or bottom position. Adding chains to the BP or squat, for example, increases the resistance during the full ROM during the lift. The higher you rise the more the extra chains are lifted, offering more weight resistance and demands to work harder on the weaker, moderate and strongest range. You do not get that full muscle involvement when just doing regular BP training. Chains do work but you may need personal hands on instruction or a very good training video to learn the proper way to use them. Not as simple as it may seem. Sometimes books only tell half the story on training.
That three phase strength curve can also be accomplished when using a power rack. BP: Setting the pins at the chest area and another set of pins four to six inches higher. These act as stops so the press does not go beyond that range, working only the weaker part of the BP. To work the middle range of the BP, use the setting of the original stops (4 to 8 inches) for the bar and place new pins at the about 4 to 8 inches higher. Again acting as stops for the middle range. For the top, which may or may not require a full lockout depending on you training needs, start from the stops on the middle ROM and all the way up. With that top position, just about anyone call use at lease 100lbs over their best regular BP. Both the chains and power rack are a form of the over load system of training. Both very effective.
If I were a full time PL'er I would use the chains when getting ready for meets. Better for speed and the natural flow/form on the BP, squat & DL's. Power Racks training is also good for raw power when the partial reps are applied and max effort used. The only negative about the power rack (if your main interest is PL'ing) is that the strength advances (and make no mistake, you can get very, very strong) does not transfer over to regular lifts that well. Like the BP, DL & squat in Pl'ing events. Good Luck.
Side Bar: Art Jones designed his original Nautilus machines based around the cam pulley and the three phase strength curve. He first made a prototype hand grip twisting device, with chains attached, for forearm size and strength. The logical next step up was using the cam design rather than the chain resistanceidea . He got the name Nautilus from the Nautilus sea shell and it's curved designed.
