I was trained to belay at a summer camp by an adventure group. The way I was taught was to always have two hands below the atc and to pull up with both hands to tighten the rope and then brake with both hands and then slide them up one by one while in the brake position. I recently started climbing at a gym where they told me that this is an unsafe method of belaying, but I’m failing to see why. My hands are always on the rope. Am I missing something? Is this unsafe?