Everything that we tied with has safety release snaps, we use lines that are slightly elastic in the trailers - they give a little. Is that what you mean? I don't know why they would put that in a lunge line but there must be a logical reason I guess.
No. You know those things guys like that are an elastic cord with a hook on each end that they can hold stuff together with? There was about 12-18 inches between the safety release and the nylon strap. It stretched. I noticed a lunge rope also had one but it was longer. The horse could probably pull back from the safety strap I would guess another foot.