A shower curtain tension rod works by an internal spring forcing the ends of the rod against the walls it's placed between. To install a shower curtain tension rod, the rod must be adjusted to a size roughly an inch longer than the space it's going to occupy. Once the tension rod is properly adjusted, it can be compressed long enough to put up, then released to stay secure.
Like all springs, the spring in a shower curtain tension rod always tries to return to its usual shape. The rod can be adjusted by using a twist point. The location of the rod's twist point varies depending on model.
To adjust the tension rod to the proper point, the owner must know how much distance she wants the tension rod to cover. This requires a direct measurement from one side of the shower to the other. Once she knows the distance to recover, she can twist the tension rod to an appropriate length. It is important that the tension rod not be twisted more than 2 inches longer than the space to be covered. Doing so requires more force to be applied to compress the rod, which makes installation and removal dangerous.