This is a rule that I didn't actually know was a thing in pazaak until I did it by accident, you can win a round if you fill your side of the board without going over the 20 points.
The easiest way to work on this is to fill your deck with cards that can give you negative values, including the ones you can switch between positive and negative as well as the yellow cards can be quite useful as they switch the values of all cards with those numbers that are on the board, including ones that were placed from the deck, just be careful though as these will also make your negatives positive thus boosting your score up.