The pilot reported that he rolled the aircraft out of a turn at 4000 feet and then the engine spluttered and stopped. Trouble checks including advice from the base station failed to restore engine power. In the ensuing forced landing, the aircraft was apparently flared too high and it bounced heavily twice resulting in the collapse of the nosegear. An engine run and inspection of components failed to detect the cause of the loss of power but a detailed laboratory inspection of the ignition switch confirmed the cause of the engine failure. This inspection detected an intermittency in the ignition/magneto switch internal wiring when subject to vibration. The ignition key had a heavy metal clip attached to it and it is believed that the inertia of this clip on rollout from the turn was sufficient to interrupt the electrical continuity within the magneto switch and cause the engine failure.