To begin threading the over-center fastener, flip the lever over the wire anchor as shown.
Take the loose end of the strap and feed it through the lower slot in the fastener.
Bring the loose end back and feed it through the top slot in the fastener.
Pull on the loose end to take up any remaining slack in the strap.
Then flip the lever over to lock in the tension.
To un-tension the over-center fastener, simply pull up on the loose end.