Humans have a deeply ingrained norm: if someone gives you something, you owe them reciprocity. A manipulator exploiting reciprocity gives something small, then cashes in the resulting obligation for something large.1
Pattern:
Example: A salesman gives you a free sample or consultation. Now you feel obligated to buy from them, even if the product isn't good. The free value was small; the expected reciprocal purchase is large.
Example: A person does you a favor unprompted. Now when they ask for something difficult later, you feel you "owe" them and comply.
Why it works: Reciprocity is adaptive — in normal social interaction, exchange of favors builds relationships. The manipulator exploits this by triggering the norm with something small, then cashing it in for something disproportionately large.
A California candy shop tested this pattern directly by handing out free samples to customers browsing the counter. Customers who received a free sample were 42% more likely to make a purchase — but the striking part is what they bought. Most didn't buy the candy they'd sampled; they bought something else in the shop entirely.2 That detail rules out the simple explanation (the sample worked as a taste-test advertisement for itself) and confirms the reciprocity explanation instead: the free sample created a general sense of obligation toward the shop, not a specific preference for the sampled item, and the customer discharged that obligation with whatever purchase was already on their mind. The gift didn't have to be relevant to what got bought — it only had to be given, because the obligation it creates is owed to the giver, not tied to the object given.
Psychology: Reciprocity is a cognitive bias; the biases page explains why this norm exists.
Manipulator-Archetypes: The Kind Manipulator specializes in this technique.