Heavy-Light power exercises uses phosphagen networks as muscle tissue contractions, and is done at maximum force. Therefore, this causes quick fatigue. Light-Heavy power exercises uses glycogenic networks through lower weights, causing faster moving muscle twitches, and faster tissue contractions (delaying the fatigue sensation). As a friendly reminder, power relies strongly on phosphagen energy networks, which uses ATP and creatine for energy. This is depleted quite quickly, therefore, it also relies on glycogenic networks, which uses stored glycogen for energy.