How does a backpack buckle click shut?

Many backpack buckles have two springy arms on one half and a hollow space in the other half. The arms point toward the space when the pieces line up.

As the pieces slide together, the arms bend inward. When their ends reach matching openings, they spring outward again. Their wider ends catch behind edges, making the familiar click.

Pressing both arms inward clears the catches so the halves can slide apart. The buckle changes shape a little to join or release the straps. It does not need a separate knot each time.