I have recently conducting and learning about Human-Centered Design. PET stands for :
All of these element could be understood by following Persuasion Design that contains 6 Principles ( reciprocity, scarcity, authority ,consistency, liking, consensus ). If you look at Airbnb.com, you would notice that they are using at least one of those 6 strategies.
-To increase the program’s user signups and bookings by over 300% per day.Airbnb used the principle of liking to reengineer their “underutilized and underperforming”.
Additionally, Persuasion is applied in many domains such as health, behavior change, businesses,etc. I would recommend you to watch and read Dr.Bj Fog books and videos. You will learn how to design application that contains persuasive techniques.This is more related to psychology, so there are many techniques that work and some don't.
I hope this is useful and if you have more questions. Please ask :).