This additional board represents the "inside" of the tile, or its "inventory"
You still make only one move across all boards, rather than one move in each board.
A piece may use any of the abilities from its inventory as its own.
Equipping your tiles with internal structures makes them more powerful.
Enemy tiles can enter your own, and attack your items directly, but enemy tiles' abilities can also be used from your inventory.
There are no limits on the depth of inventories.