+1
Tensioners are for keeping a tarp taut as the tarp material "relaxes" from the initial setup configuration, whether from soaking up rain, temperature fluctuation, wind gusts, etc. not for absorbing wind gusts or anything of that nature.
Also, the shock cord has a substantially lower breaking point than most pullout cordages so it provides a fail safe if you trip over a pullout. Tieout cordage is stronger than the tarp material so if there was no shock cord in the mix, your tarp would rip before the cordage in such a situation.
Bookmarks