The YuXin 3x3 Treasure Box is a 3x3 with a compartment in the center where you can store small personal items.

In order to open the compartment, simply get the puzzle in a solved state and push down on the white center cap. Once it locks into place, unscrew that layer. In order to reassemble the puzzle and reset the center cap, screw the layer back onto the puzzle securely and pull gently on that layer.