The "localized" growth that they think they experience, is more likely a result of the muscle just recently growing (perhaps from a change in movement) and them attributing the increased size in the desired area to the excersise targeting the area, rather than the fact that, most likely, the whole muscle grew.
My comment about load has nothing to do with powerlifting type weights. Even with the weights used in bodybuilding, or even rehab, you're going to end up recruiting fibers from the whole muscle.
Can you show me a bodybuilder who has successfully changed the shape of one of their muscles without implants or synthol?