App note from Vishay on the impact of torque on thermal resistance of TO-220 devices. Link here (PDF)
When the TO-220 was first introduced, most applications required something less than the full power handling capabilities of this package. Hence, the TO-220 is almost taken for granted in terms of its excellent power handling capacity and ruggedness. Today, however, advances in semiconductor technologies are bringing application demands closer to the TO-220’s capabilities, so an understanding of these is more relevant than ever.