The elegant simplicity of a black hole is encapsulated in the famous “no-hair theorem”, which tells us that an astrophysical black hole is completely described by just its mass and spin angular momentum (the third parameter, electrical charge, being effectively neutral in astrophysical settings). A spinning black hole is an enormous capacitor of angular momentum, with the spin as a ready energy source that can be tapped mechanically in a black hole’s ergosphere. Spin has long been proposed as the likely energy source behind the enormously energetic jets emitted from black holes (e.g., Blandford & Znajek 1977). This assumption has gained recent support both theoretically (e.g., Tchekhovskoy et al. 2011), and observationally (Narayan & McClintock 2012; Steiner et al. 2013; McClintock et al. 2013, but see Russell et al. 2013).