When the fastener is “open” you can feed the excess webbing through the fastener. When all of the slack is removed, the act of “closing” the fastener will add tension to the webbing and hold the tension tightly in place.
When the fastener is “unlocked,” you can insert the extra webbing through it. When all slack has been eliminated, locking the fastener will tighten the webbing and secure it firmly in place.