While tricksters have been testing their balance by walking on tightropes for centuries, slacklining is a relatively new practice that was developed by a pair of Yosemite climbers in the 1980s. In recent years, rock climbers have popularized this activity of focused balance.
What separates a slackline from a tightrope is the dynamic nature of the line. A tightrope is a taught tension cable, whereas a slackline is made from one-inch webbing slung between two trees, posts or parked cars and made taught with a simple friction method. Some prefer the line taught, others like it a bit more slack.
Suffice it to say, this shit is as hard as it sounds. But progress comes quickly if you're focused and determined to get back on that horse. Intermediate slackline walkers can walk the length of the line (keeping your line in the 30-60-foot length makes adjusting the tension more manageable), turn around and walk back - a practice known as "full manning" (climbing terms like "on sighting" and "sending" are also used to better describe one's efforts). Dropping into a sitting position or a lunge are also tricky intermediate moves. Laying down while casually propping your head with one arm and taking a sip of your bronson with the other requires a much more advanced skill set.
If you're really looking to raise the bar, try highlining, which is exactly as it sounds - slacklining up high. Whether it's done 20 or 200 feet above the ground, rest assured it has been done. More than 20 years ago, Adam Grosowsky pioneered a highline at Smith Rock, running a line from the mouth of Monkey Face to the Diving Board, the prow just south of Monkey Face. And he did so "free solo," or without any protection securing him to the slackline. You fall, you're dead. Generally speaking though, most highliners wear a climbing harness and attach themselves via a tether and a carabiner to the slackline, making for a six-foot fall, versus, say, falling to your death.
Obviously, you can have a good time honing your balance while calling on all sorts of little stabilizing micro-muscles you didn't even know you had, without risking death. You will, however, risk twisting your ankle, breaking a collarbone or bruising your ego - much to the amusement of your buddies.