I love this movement too. It will find any weaknesses you have in your hips and make you very aware of them! Here's video from Mike Boyle's site again. He teaches the movement holding only one DB, which is the way I learned to do it. NR4W shows two DB's. I don't feel strongly about whether you hold one or two DB's. This video is excellent.
You want to keep that long straight line from the top of your head to your heel. Think of it as a pendulum (remember those wooden woodpeckers that tipped their nose into a glass of water? like that!). The supporting knee is soft. Keep your weight on your heel. Reach back with the back heel. Keep your spine neutral and your chest up. You should feel the work in the glute and glute medius of the supporting leg, especially on the return to standing portion of the lift (the concentric contraction) if you've kept your balance. Remember that balance is assisted by breath control and controlled movement.