What is an airpot? (stirling engines)

Hi, I am currently trying to create a lamina flow Stirling engine as a little project on my own. I understand the basic concept of Stirling engines and the fact that they rely on the contraction and expansion of air but i noticed some stirling engines (lamina flow) have airpots. I would like to know what these are and what their purpose is. an example of an airpot being used is in the picture below. Thanks for any help!



(its the cylinder between the heat source and the piston.) My guess was it narrows the cylinder so hot air is then pressurized has a better chance at pushing the piston out?