Hi Rom
I've used the same technique for my BC. I would use it when we were just standing in class whilst the instructor was talking. I would drop treats from standing position between his front feet, reinforcing that him lying there was what was required, rather than fidgeting around.
The "randomness" of the treat dropping kept his focus. I found that when I kept bending down to give him his treat for holding a down, he would break from the position.
I now have an extremely reliable down, so reliable that when he's holding a sit he goes into a down trying to win a treat! I'm back to rewarding a sit and reducing the rewards with the down...........just to let him know that sits win treats too, when that is what is being asked.