# Photographer presses the shutter release button halfway.
# Ambient light metering of the scene is conducted.
# Photographer presses the shutter release all the way.
# The master flash unit sends a wireless signal to all slave units in group A, instructing them to issue a low-power preflash.
# Any slave units in group A fire a preflash and the camera records this light output using its evaluative meter.
# The master flash instructs group B slaves to issue a preflash.
# Any slave units in group B fire a preflash and the camera records this light output.
# The master flash instructs group C slaves to issue a preflash.
# Any slave units in group C fire a preflash and the camera records this light output.
# The camera calculates what the final flash output for the scene should be, based on both the preflash data from each slave group (if any) and the user-defined group ratios/flash exposure compensation settings.
# The camera flips up the mirror and opens the shutter.
# The master flash instructs all slave units to fire simultaneously.
# All slave units fire at whatever level the master unit has told them to. Naturally if the master flash unit is flash-capable (ie: not an ST-E2) and is configured to fire then it too will do so.
# The camera flips down the mirror and closes the shutter.