To understand flash sync, you need to know how a focal plane shutter works. Basically, it runs at just one speed, and different exposure times are created by altering the duration between when the first curtain runs down over the frame, and the timing gap before the second curtain follows. Google it and you'll find a few graphics to explain.
What this means is that at higher speeds, above 1/250sec on a 40D, the second curtain starts to cover the sensor again before the first curtain has reached the bottom. If a flash is fired then, you get a dark band across the picture. The higher the speed, the bigger the dark band. Only at 1/250sec and longer is the whole sensor uncovered at one time.
Bear in mind that the flash duration is very short - anything between 1/1000sec and 1/30,000sec depending on the power output. That's how the camera is able to squeeze it in to the tiny 'window' when the whole of the sensor is uncovered by the shutter.
From this you will also see that there are indeed two exposures happening simultaneously - the ambient exposure which is the shutter speed set (1/250sec or longer) and the flash exposure which is much shorter at 1/1000sec or less.
To get over x-sync problems, the 430EX has a high speed sync mode which alters the way the flash fires, and pulses it very rapidly. It is able to keep this pulsing going long enough for the shutter to complete a full cycle, regardless of the timing of the curtains, so it works at high shutter speeds where normal flash cannot. It's not actually flash in that respect, but continuous light - a bit like switching a room light on when you press the shutter release, and turning it off again immediately after. The downside is that HSS wastes most of the light, which falls on the shutter curtains instead of the sensor, and therefore range is substantially reduced. It is very handy for close shooting in bright sun though, for fill-in flash etc.
Some folks seem to think that second curtain sync is some kind of magic bullet with slow-sync flash (so-called 'dragging the shutter'). It is not, and usually it is best avoided. Second curtain sync is useful only when a) you have a long shutter speed to get some ambient light in there, b) there is movement which creates ambient light blur, and c) the direction of that movement is important (it usually isn't). The downside of using second curtain sync when you don't need it is that it makes the timing of the flash less predictable so you can miss out on expressions or actions.
