The problem is the duration of the flash.
It's around 1/100000 of a second so that means it needs to expose the whole sesnor in that time.
The problem is we use a shutter with DSLR, i can explain how that works but it's already written down so i dont need to.
Focal-plane shutter - Wikipedia, the free encyclopedia
So basically you've 2 shutters and a leading and trailing one.
When the first one starts to move the exposure is started and the moment the second shutter start to follow the exposure, so the time/distance between the two shutters is the shutter speed.
Now since the flash exposure is so short the sensor needs to be completely exposed so the first shutter needs to be fully down and the second shutter still needs to start moving, and this happen with Pentax DSLR at 1/180 of a second
Now for the rest whats being said here.
vonBaloney - high speed sync mode, special mode the flash strobes instead of flash so you can use higher shutter speed but the flash exposure is longer.
sterretje - the camera keeps continue metering like normal and simply add the flash more as an after thought
anvh - trailing shutter, instead that the flash flash at when the first shutter is down, the flash flashes when the second/trailing shutter start to move down to finish the exposure, you get a different lighting effect then.