That's a guess, but I think you're right that it's a matter of how far you fall before the lifeline "catches" you, which should be as short a distance as possible. It might translate "The lifeline/lanyard is elastic, so the free fall distance should be kept to a minimum to avoid the risk of hitting the ground".

Check out these quotes:
"Keep lanyards as short as possible to reduce the distance you could fall. Arrange the lanyard to limit a free fall to no more than 1.2 m (4 ft.)." (http://www.worksafebc.com/publications/Health_and_Safety_Inf...

"48-inch stanchion height reduces extended free fall distance" (http://www.millerfallprotection.com/miller/skywalker/skywalk...