http://www.harrisonrods.co.uk/Testcurve.htm
What is Test Curve?
Test curve is a guide to rod power and action. It is a function of both, and it is usually taken to be the weight in pounds and ounces needed to pull the tip of a rod round to 90 degrees to the butt. It is obvious why test curve was adopted. Just like length of a rod, it is handy to have a measurement that can tell you something about the power of a rod. It can be measured in a number of ways, but simplest is to imagine a rod, clamped by its handle in a horizontal position, a few feet off the ground. With line threaded through the rings, you attach a weight close to the tip ring. Start off with 1lb on a rod assumed to be say 2lb test curve. The rod will now bend under the weight, the tip moving lower, and a curve developing throughout the rod. To measure test curve you simply add weight until the tip is at right angles to the butt section. Another option is to use a spring balance to load applied as you increasing the pull at right angles to the butt. So the test curve of the rod equals the weight applied, the load, to bring the tip to 90 degrees from the butt.