Electronic flashes work by charging up a capacitor with electricity, then releasing the stored-up power in a split-second burst of light. This charging process, the "recycle time," takes up to a few seconds on larger units - which can be a problem if you need to take several flash photos in fairly rapid succession, such as at a wedding.
Many EOS flashes have the ability to be triggered even if not fully recharged, on the theory that there are times when you're better off being able to take a photo without a full flash charge available (ie: at a lower guide number than maximum) than having the flash not fire at all. Flash units capable of this feature have a two-colour flash ready ("pilot") light. If the light is red then the flash is fully charged. If it's green then the flash is only partially charged but will still fire anyway if you take a photo.
It can be quite frustrating using a flash unit without rapid-fire, in fact. It's all too easy to take two photos in succession only to find that the second one didn't trigger the flash and so is totally underexposed.
Rapid-fire mode will not work if the camera is in continuous film winding mode, if the flash is in manual mode at full or half power or if the camera is in stroboscopic flash mode at a fairly fast setting. The 430EZ does not work in rapid-fire mode if an external battery pack is used.
Flash units with rapid-fire capabilities:
Speedlites 160E, 300EZ, 420EZ, 430EZ, 540EZ, 550EX, 480EG. Flash units with no rapid-fire capabilities:
Speedlites 200E, 220EX, 380EX, 420EX, ML-3, MR-14EX, MT-24EX, 300TL.
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