Can someone explain this? (Thermodynamics)

What a bizarrely interesting diagram. It's a schematic and a graph intergrated in one illo.

 

Well. . The exact process is happening in the Cylinder. If the Piston press in thats the compression comes in. and i think the process is Isochore.

 

You should not expect PF helpful staff to explain thermodynamics to you, you should
1.read a chapter on thermodynamics in a physics books
2. read the section on thermodynamics at physicsclassroom.com or hyperphysics.com , both are great sites
3. if you have any questions about thermal dynamics please ask them here after you have done some research on it.
But basically it has to do with differences in potential, Hot Air goes to wear cool air is.