The limited dexterity that existing hand prostheses provide to users contrasts with the manipulation abilities exhibited by state-of-the-art robot hands. This paper presents an underactuated robot hand with in-hand manipulation capabilities to demonstrate the use of underactuation in the development of effective hand replacements. The paper describes a specific underactuated hand architecture, representative of many existing underactuated hand prototypes. First, the hand is modelled and its ability to manipulate objects of different geometries is analyzed. Second, a manipulation strategy suitable for prosthetic applications is proposed. The strategy enables the hand to manipulate objects in-hand without any a priori information of their geometry or physical properties. Finally, experimental tests conducted to validate the theoretical results are presented.