Convential electric start systems use power from a battery as the source of energy. A battery with sufficient energy for starting large engines would be excessively heavy. The 787 uses new technology, not available years ago. The engine mounted generators can also perform start motor operation. They are variable frequency machines that can be driven by the generators on the APU, or the other engine. The starting operation would be similar to a bleed air system except that AC power through wires is used in place of bleed air through ducts. The initial start power would come from a battery to light off the APU, followed by much higher capacity electrical power from the APU to turn the engines.