It consists of an eclipse's View where you can evaluate xpath expressions in the current opened document.
This view also has an "Auto Evaluate XPath" button that switchs on and off the ability to evaluate the expression while you are editing it.
A history entry is saved in the expression combo.
Either you use automatic evaluation or not, only valid results will be added to the history entry.
History can be cleaned using the "clean" button in the view's toolbar.