Moving Safari window

Sometimes when I mis-click on a bookmark in the upper part of my Safari window, just below the address field, I am able to move the entire Safari window around on the desktop without holding down the trackpad. As if it is locked.

Nice little feature, but I can't for the life of me repeat it consistently.

I double tap, I click, I ask it nicely, I look away as if it were shy, but nothing works.

So, does anyone know the secret trick?

Select Dragging and Drag Lock in System prefs Trackpad.
Use a gentle double click on the top of the safari window ~ a firmer one will minimise it to the dock.
After dragging the window out of the dock a few times you get it right