Many people who see a highliner on a highline for the first time, think that the forces are very high, especially when falling into the highline (leash fall). Since the slack ("sag") in highlining is not as limiting as in longlining over ground, most highlines are much less biased and this leads to lower forces than you would like.

Check out this video and write out of the tests we did:

 

Write-out

We wanted to measure the forces that occur in highlines. For this we rigged 3 different highlines and put an rockExotica Enforcer on one end. 
Two on pinkTube (18% stretch @ 10kN) and one on halfMarathon (4% stretch @ 10kN)

Parameters 63m
pinkTube 

39m
pinkTube 

41m
halfMarathon 
Weight of slackliner 57kg 57kg 57kg
Standing tension 1.8kN 1.4kN 1.7kN
Tension with Slackliner 2.4kN 2.1kN 3.1kN
Max. force bouncing 1.6kN - 3.6kN 1.3kN - 3.3kN 1.6kN - 4.4kN
Hardest leashfall 4.3kN 4.0kN 5.3kN
Bouncing speed  0.35 Hz 0.46 Hz 0.56 Hz
 
 
Some conclusion were made: 
  1. Leashfall on the lowest point of the line creates less force on the line then intense bouncing. 
  2. Walking towards the anchor with the force measurement device meant an decrease in the force. 
  3. Polyester lines on short lengths puts more force on the anchors and the body of the slackliner during leashfalls.
  4. A heaver slackliners puts more stress on the anchors during a leashfall.
  5. Strechier webbings take longer to absorb the forces of a leashfall or bounce. 
  6. The shorter the line, the quicker the bounce. The tension also seems to influence the speed of the bounce.