This gimmick is a revolutionary concept in magic for productions of items. As with your standard "thumb tip" style techniques which you're already familiar, you can produce, transform or vanish many items. These could include large silks, a stack of coins, large quantities of salt, and several live goldfish!
On the stage, this gimmick is invisible, yet you have secret container in your hand. During close-up work, it's also your powerful weapon.