How does double-tap to drag work? I'm using an image editing program, and I'm trying to do a large clone.  This requires me to keep the mouse button down (virtually, this is on a trackpad) for a long period of time, and any mistake means I have to start all over.  (Not to mention, if I get to the edge of the trackpad and still need to push the pointer further I am in trouble!) So I have been looking for a way to keep the software thinking the button is down without having to keep my finger touching the trackpad for an extended period of time.

I read this answer here, which seemed to offer a solution to the problem.  However, perhaps I am not understanding what it really means to "double tap", because after setting the option exactly as given in the answer, two literal taps on the trackpad in quick succession do nothing to enable dragging.  When I do it in my image editor, I see two modifications made to my image, as if I made two separate taps.  If I try it on the top of a window where I would normally click and drag to drag the window around, dragging does not start and the taps are completely ignored.  I already have single tap to click enabled, so I know a tap is being registered properly.  Setting the "Enable Dragging (with Drag Lock)" seems to have had absolutely no effect.

Is there something else I am missing?
After some experimentation, I discovered that it isn't quite a double-tap that is required to activate the drag lock.  The second tap of the double-tap need to be held down until the drag is started instead of being released immediately.

So the sequence of steps look something like this:

touch trackpad
release after ~0.1s (or some short period of time)
wait ~0.1s 
touch trackpad
begin drag
release touchpad


And then when I am done dragging:

touch trackpad
release after ~0.1s