The pak sao is a forcefull press or smack that traps your opponent's arm. The images below show a single pak sao and simultaneous palm strike and a double pak sao followed by an elbow. Both combinations can work very well in self defense and in sparring. The pak sao can also be used as a defense against straight punches.
Notice how blue shirt keeps his chin down when using the single pak sao and palm, staying covered against gray shirt's potential cross. For more on staying covered while trapping see my page on wing chun sparring integration.
The advantage of the double pak sao is two-fold: First, because both of your opponent's arms get trapped, he'll be unable to hit you with the rear hand. And second, he'll have a much harder time defending against your follow-up strikes. The double pak sao is requires you to get closer to your opponent than the single, so knees and elbows are good follow up techniques.
See more wing chun techniques.