Free Free Run in FEM analysis is a common method to check in Ansys and other FEM solvers whether the FEM is mesh modelled correctly without any free edges or missing connections.
Free free Run solves the model without fixing constraints, so that the whole model will be floated in space .
If the model is meshed correctly, then if you run the free free run, then the complete mesh model will move in X, Y, Z respectively for first three mode.
Incase if the mesh is not done properly, some connections are missed, then some parts will be hanging seperately when you animate the free free run mode shapes. Based on this observation , you can fix the model.