Overslack is a slackline simulator/game.
The simulated world has only two dimensions. Although the forward direction offers many possibilities on a real slackline, it has very little to do with balancing and is quite uninteresting to simulate. Slow speeds are assumed, so the air resistance is neglected. The time evolution is calculated by applying Newtonian physics numerically as many times per second as the machine can handle.
The speed of the simulated world is configurable, making it run five times as slow as the real world makes it slow enough to be manageable and fast enough to keep the attention up. Unlike normal grass ground, the ground in the simulation is bouncy and doesn't cause injuries. It is (unfortunately) a bit unrealistic, but makes it more fun.
The simulated line behaves according to a quite simple model. An ideal spring between the line's point of equilibrium and the slacker's feet makes a good approximation, but an ideal spring gives back all energy it stores, which would cause you to bounce forever without helping with your legs. To simulate some friction, it has a somewhat different spring constant depending on whether it's extending or compressing. It is of course fully configurable, so anything from an ideal spring to an old, tired and non-elastic line are possible to simulate.
The slacker is controlled with the mouse. Move the mouse down to extend the legs, and move it up to crouch. Move the mouse to the right to lower your right arm and raise your left, and to the left to do the opposite. This is pretty much all you can do on a real slackline too, except that you can move more than just your arms, but that's just a question of terminology.
The slacker is infinitely fast and strong, if you move the mouse the slacker will move instantly to the new position regardless of the force and energy required.
It is possible to jump off the line and land on it again, with or without bouncing on the ground in between, but to make it simpler the slacker is moved to a perfectly balanced position whenever you press the right mouse button.
In addition to the line, slacker and some on-screen numerical physical information, vectors visualizing location (white), speed (green) and acceleration (red), both translational and rotational, are shown.