question about dynamic meshes
 

I am trying to simulate a piston-cylinder model using dynamic meshes of FLUENT but not the "In-cylinder" method. The model is 2d and includes a rectangle face with three stationary walls and one moving wall. When the wall moves, the mesh before the moving wall are split or merged when the conditions are met. My question is why the meshes before the moving wall are compressed or expanded instead of being split or merged?? Is there something in the "dynamic mesh parameter" I don't know?? Thank you for any help!!