It seems that I had to click in the upper left hand corner of the controller point boxes to be able to drag them.
if you right click on the boxes, and have a look at their properties, it's possible to set the value for selection area.
The other issue is that it's
possible to drag those guys outside the canvas (at least temporarily, they snap back inside), causing it to provide values outside the the 0->1 range.
(in) | [moses 0] | [moses 1] | (out)
sorted!