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It’s basically a climbing rope taped below the slackline. You are attached to both the slackline and the safety rope with your leash, connected by a steel ring that slides along the line. If you fall, you hang below the line. If your slackline snaps, you might slide a bit towards the middle of the rope if it has more slack.
So, yes, it’s rather safe. The biggest risks is falling close to the edge (where you can hit your head against something), and setting up your anchors correctly.