The H.S.I. does indeed utilize a gyroscope (directional) and usually electrically powered, to provide a steady reference direction. Like the D.G. the reference direction is subject to precession and transport errors. The gyro must also be aligned either manually or automatically with magnetic north. A mode selector on the panel in the vicinity of the display (mechanical or EFIS) controls the source of magnetic input to the H.S.I. In the 'auto' mode directional information is provided from the flux detector. In the 'manual' mode the indicator may be slewed to realign with the standby compass system - a switched version of a D.G. Hence when used in conjunction with a C.D.I. the information is correct in either mode.
Some stand-by H.S.I's. have vacuum powered gyroscopes using the bleed air system to generate the vacuum.