Third, emitting the pre-flash uses some of the energy stored in the flash capacitor, reducing the power of the flash for the main flash a little bit. They just flash the flash, and stop the camera down to a pretty good aperture, and hope for the best. This usually results in a narrower distance range for good exposures. 35mm film cameras can measure the flash in a real TTL fashon, by measuring the light bouncing off the film, sometimes even with a matrix arrangement. This isn't possible with CCD cameras, there is no film, so one of the other techniques has to be used.
Some cameras rely on a sensor on the flash itself.
This is usually called program mode, and is very similar to the pseudo TTL mode on the Coolpix 950 and 990. Since the sensor is just a wide area light gathering sensor, it is easy to fool too. Underexposure due to light background. This is a very common problem. A common example is a person standing against a white wall, or an object sitting on a light colored rug, with the background close to the object.