The key point to these questions is what happens in the reverse, or "what takes place when energy is removed from the system" and "where is it going". Since this is the norm, it would make sense to study these actions first, then devise a method that does the exact reverse to achieve a gain in electrical energy.
In general, in a reactive circuit, if there is no resistance, there is a phase angle of exactly ±90° between the vectors of voltage and current, this represents Circulatory Energy and not necessarily Power as we usually understand it. To describe this specific situation, we use the term called "Reactive Power".