From my understanding you deload to prevent injury and give your body a rest. It's an active week of "rest" in which you decrease weight, increase reps, not training to failure. You would de-load when gains start to slow, or stall, as to prevent over-training. With that, I've noticed my bench after 8 weeks of heavy training, has started to slow down, as for as gains go. So should I deload my chest? And as far as I can gather, you can deload one body part at a time.