  Recent general relativistic MHD simulations have suggested that relativistic
jets from active galactic nuclei (AGNs) have been powered by rotational energy
of central black holes. Some mechanisms of extraction of black hole rotational
energy have been proposed, like the Penrose process, Blandford-Znajek
mechanism, MHD Penrose process, and superradiance. The Blandford-Znajek
mechanism is the most promising mechanism for the engines of the relativistic
jets from AGNs. However, an intuitive interpretation of this mechanism with
causality is not clarified yet, while the Penrose process has a clear
interpretation for the causal energy extraction from the black hole with
negative energy. In this paper, we present a formula to build physical
intuition so that in the Blandford-Znajek mechanism as well as other
electromagnetic processes, negative electromagnetic energy plays an important
role to extract the rotational energy of the black holes causally.
