The pilot reported that he was flying at about 100 feet above ground level when the engine failed suddenly and completely over thick scrub. He decided to stall the aircraft into the foliage canopy, but the aircraft did not make contact with the main canopy other than small branches, and impacted the ground at a 60 degree nose down attitude from a height of about 40 to 50 feet. The nose section, undercarriage and fuel tank were totally destroyed on impact. Examination of the engine has not revealed any mechanical defect. The pilot believes he may have inadvertently bumped the "engine kill switch" to the off position with his knee. The Bureau has been advised by the Australian Ultralight Federation that this particular switch has since been fitted with a shroud to prevent inadvertent operation.