After watching the drill, the question remains, how do you "create" a steady dog? The usual replies focus on making the dog sit. Afterall if he remains on sit, the problem is solved. However, there is a catch to the expectations of a solid sit. The most important one is the dog must be focused (somewhat) on the handler. This means being responsive.
If you noticed in the video, the dog is expected to heel off lead changing directions (left, right and backwards, moving at different speeds), put in a down position and "waiting" for the handler to "go about his business" (picking up spent popper casings, replacing ducks near the wingers and loading poppers for the next pass).
The dog is focused on what the handler decides to do next because the session is not just about retrieving ducks.....to repeat.....it's all about what the handler decides to have the dog do next. The dog knows this is the primary "expectation".
The point is a steady dog is always aware of who is in charge. The dog did not become steady just because sit was "hammered home". Sit is more of a command to be ready for what is next. It's called being responsive.
What is not seen in the video are the simpler teaching sessions which give more foundation to the standard. A dog that sits and then thinks "OK, what's next? is very different from one that has done the "token sit" and is a blink away from being self-employed.
There are early training drills which focus and control a dog's forward momentum through the handler. The prep/teaching OB drills provide an entirely different perspective of "sit" for a dog.
Here's a link to one such technique develped by Pat Nolan. I found his approach workable and extremely effective.
Steady Retriever - Approach