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:
The forces that occur during slacklining are composed of the preload force and the weight of the slackliner. This can be both static and dynamic. It is not possible to determine the forces without measuring device exactly - but one can calculate the forces in static slacklining with a formula approximately.
Here are some basic rules of the forces: