The PAT, or Point After Touchdown, is a special circumstance. Time will not expire from the clock, for example if a team is down by six points and they score on the last play, that is to say they score with zero time remaining on the clock, they are still allowed to kick a convert, or try for two as the case may be.
The defending team is allowed to score during this time, but they are only allowed to score the two available points. Many situations can lead to this result. If a team attempts to score a two point conversion, they can do so by either running the ball into the end zone or passing the ball into the end zone. If the ball carrier on a run fumbles the ball before entering the end zone, the defending team may recover the ball and return it for two points. It's the same situation if the defending team intercepts the ball, whether they intercept it in the end zone or outside of the end zone. If a team intercepts the ball in the end zone during regular play (not a PAT) and they are tackled, the intercepting team keeps the ball and gets to continue from the twenty yard line, this is called a touch back, same deal if they run out the back of the end zone.
If a team attempts to kick a single point conversion and the kick is blocked, the defending team is allowed to recover the ball and return it to the other end zone for two points. Essentially either team could score two points during PAT.
For PAT records check this site out.
For way too much information on the PAT check this site out.
