Briefly about manipulation from the book "Psychology of Influence" by Robert Cialdini

Reciprocity rule. Explanation: the manipulator offers you a "gift", after which he offers to buy something, or perform a certain action. The automatic reaction forces you to agree to his offer, even if the amount of your expenses is clearly inadequate for his gift. Example: Hare Krishnas on the street will not ask for donations directly - they first give you a flower or a book (absolutely unnecessary for you), and then they say that in return you can thank them by making a donation.