Paralysis is a type of damage that, as you might have guessed, paralyzes the target. The number of combat rounds the paralysis lasts (the duration) is determined by the number of paralysis points done and the target's size.

If that is all the paralysis damage taken, then the target is only paralyzed for that many rounds. However, if the target takes additional paralysis damage, a new duration is calculated, and the target takes half as many points of damage as the lesser of the new or old durations. In other words, if the target is already paralyzed:

The new duration will be the greater of old and new duration (not cumulative), so if the new attack has a longer duration, the target will be paralyzed for longer.