I think he means they fold back in after tightening. They're known as swing-back blades, as centrifugal force makes them swing out in place whilst the engines running, but if the blades strike something solid such as a rock, concrete etc, the blades swing back and reduces the impact-damage.

Least ways it sounds about right laugh

Dirty Berty's Lawn & Garden Care
Servicing the Moreton Bay Region
www.dirtybertys.com