A forced landing is a landing by an aircraft made under factors outside the pilot's control, such as the failure of engines, systems, components or weather which makes continued flight impossible. For a full description of these, see article on emergency landing. However the term also means a landing that has been forced by interception.
A plane may be compelled to land through the use, or threat of use, of force, if it strays off course into hostile foreign territory. The customary procedure is for the military plane to approach the airliner from below and to the left, where his plane is easily visible from the left seat where the captain sits. The forcing plane waggles his wings to signal the demand for a forced landing.
... aircraft that fail to identify themselves, enter the airspace without a necessary permission, deny to follow a prescribed route, head towards a prohibited zone, or violate of a prohibition of flight may, by strict observance of the relevant standards and procedures, as a last resort, be intercepted, identified, escorted to the adequate route or out of the prohibited airspace, or forced to land by military aircraft of the territorial state.
^... the international requirements for making an intruding plane follow an air-force escort to the ground – moving in front and to the left, where the civilian pilot can see the escort, and waggling the fighter's wings. The Washingtonian November, 1985 – Page 153-154