Canon Speedlite 430EX Instruction Manual page 29

Setting the Slave ID
Set the slave ID if you use two (A and B) or three (A, B, and C) slave
groups.
Modeling Flash
If the camera has a depth-of-field preview button, press it to fire a 1-sec.
burst of flashes. This is the modeling flash.
You can check the lighting and shadow effects. You can fire the
modeling flash for both wireless and normal flash shooting.
Do not fire the modeling flash more than 10 consecutive times. If you fire the
modeling flash 10 consecutive times, allow the Speedlite to rest for at least
10 min. to avoid overheating and deteriorating the flash head.
The modeling flash cannot be fired with the EOS 300/REBEL 2000 and
Type-B cameras (p.2).
About Slave Group Control
Slave group A
ID = A ID = A
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Press the <G> button
1
so that <x> blinks.
Press the <9> button to select
2
the slave ID (A, B, or C).
Press the <8> button to set the
3
slave ID.
For example, if you have the slave ID set
to <1> for three Speedlites, all three
Speedlites will be controlled as if they
were one Speedlite in slave group A.
ID = A