How does double-tap to drag work?
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