A slackline is commonly constructed with three sections of one-inch : a long section of webbing (30-100 feet) strung tightly and connected to the two shorter sections (8-12 feet) that are called "tree slings" and are used as anchors on either end. When using trees as anchors, padding (aka "tree friendlies") can and should be used between the slings and the trunk of the tree to protect the trees and avoid fraying the webbing. The padding usually consists of cardboard, carpet scraps, or branches placed around the trunk of the tree to distribute the pressure of the webbing over a greater surface area. The most difficult and widely discussed element of a slackline setup is the tensioning system. Common setups include simple friction methods, using wraps of webbing between two , a , a comealong, a carabiner pulley system, a roped pulley system, or a commercial slackline kit.
Slacklining is a bit like walking a tightrope only lower to the ground (and the line is slack not tight). It is reputed to help core balance, mental concentration and smoothness of motion. I think it would improve strength and agility too. It isn't easy to learn so you may need a bit of 'stick-at-it-ness!' but it is quite addictive! We found this was a fun sport for the whole family and one site claims to have taught people between the ages of 3 and 84!!