A Flexicurve isn't really a system. It's a very bendy measuring tape, that's all. It can easily be pushed and twisted or bent, etc, to conform to the horse's body with just your fingers. They only cost about $8-$10, and they're nice for saddle fitting because they have a measuring tape on one side. The links I posted were to show how to do a withers tracing (whether you use the Flexicurve or any piece of wire) and the templates on the other site are free downloads.
You can use it for other stuff that's hard to measure, too...like your hat size, your calf size (for those ridiculous english boots that seem to fit everywhere else but there), and thigh size (to make sure your chaps fit) LOL