You have a power stockpile that stockpiles the excess energy from your powers. Any time when a power isn't actively being used at 100%, it instead fills this stockpile with energy that can later be used to boost your other powers. Purely synergistic powers that don't do anything on their own don't give energy, as they don't have any mass excess that could be utilized. The excess energy gained from a power is proportional to both it's SP cost and how much of it is currently unused, and the same applies to the costs when boosting a power. This isn't perfectly efficient, causing you only to gain half as much energy as you should.