Extend Edge

I have been using "Extend Edge" for a while, but have only ever been able to use it for snapping to polygon plane.
In the tool description, it states:-

"Extend Selected edges to a polygon plane or mesh surface
Move cursor over to highlight a polygon if polygon snap, or a vertex/edge if mesh snap"

But selecting a vertex or edge does nothing.

So how do I get "Extend edge" to extend to a mesh?