Overcenter straps are constructed using an overcenter buckle rather than a ratchet or cam buckle. Overcenter tie downs are a great solution for quick cinch applications. Overcenter buckles don't ratchet tight, so they wont create the same tension as a ratchet. However, they are frequently used to hold down tarping on the sides of trailers.
When using an overcenter buckle, the adjustable strap is wound into the buckle, then the buckle lever is clasped closed. Proper use of the buckle and winding procedure is critical for a secure load.