I was working with the trainer at the gym a couple of days ago, doing my 15-minute HIIT before going to work, and I remarked how the exercises were so much harder because there was almost no pause at all between sets.
He replied that this allows many more of the muscle fibres to be recruited into the effort. The reason he drops the weight between sets is to ensure proper form during each movement. There’s no point in doing the exercises if you’re not doing them right.
I’ve read elsewhere that an advantage of the intensity of the training is that it totally depletes the glycogen stores in the muscles, requiring that the body replenish them rapidly. This means that you’re dropping your blood sugar levels and triggering the generation of additional glycogen from either your digestive system or your fat stores. What’s certain is that if your muscles are burning all the energy in your body and you’re on a low-GI diet, there’s no surplus for your body to store as fat.