Originally posted by sterretje It's not related to automatic versus pTTL, both modes work the same way from that perspective; the only difference is the condition under which the the switch is switched off (instructed by camera in case of pTTL mode, decided by flash in automatic mode).
Sorry if my explanation was confusing, but this isn't 100% accurate either. In auto mode, you either set the aperture on the flash to match what you're using on the camera (or creatively lie to it) for off-camera work, or the flash can read the aperture directly if it's on the hotshoe. So let's say you're shooting at f/8, the flash will look at the scene when the shutter is pressed and begin emitting light. It will keep emitting light until your subject is correctly exposed for f/8 and then it will abruptly stop, and any unused power gets recycled for the next shot.
That's how TTL worked too, except the camera made the judgment call, and quickly relayed a signal to the flash to turn off, but that's not how pTTL works. pTTL uses an underpowered preflash to judge the correct exposure. You press the shutter, the preflash fires, the camera evaluates the scene and tells the flash, "ok, discharge X amount of light", the shutter opens, the flash does as instructed, the shutter closes. pTTL decides before hand how much light is to be emitted, there is no adjustment on the fly.
Most of the time the difference in the two systems is purely academic, but there are times when one system has the advantage over another. If your subject is moving, then Auto mode is a big advantage, because from the time the preflash fires to the time the exposure happens the flash-to-subject distance changes (not to mention the exposure is delayed because of the preflash). Also, in my experience, auto mode does a better job in scenes with strong back lighting, such as fill flash for a portrait that includes the sun in the frame. If you're using macro tubes or filters on the lens though, pTTL is probably the better system.
Here's a great explanation of all the different types of flash control systems.
FlashZebra.com: Flash Exposure Control Methods