Slack is extra – extra line in the rope. Slack is not keeping things tight. It means not pulling, or at least not pulling so much that you use up all the slack.
In animal training, slack is a reward. When a horse does what you want, you give it slack in the reins... (CONTINUE READING HERE - just copy and cut in your browser. Wasting too much of my "slack" time trying to figure this out.. )