The invention relates to an architecture of a propulsion system of a multiple-engine helicopter comprising turboshaft engines (5, 6), characterised in that it comprises: at least one hybrid turboshaft engine (5) that is capable of operating in at least one standby mode during a stable flight of the helicopter, the other turboshaft engines (6) operating alone during this stable flight; an air turbine (30) that is mechanically connected to the gas generator (17) of the hybrid turboshaft engine (5) and is suitable for rotating said gas generator (17); means for withdrawing pressurised air from the gas generator (27) of a running turboshaft engine (6); and a duct (31) for routing this withdrawn air to said air turbine (30).