It’s physically easier than just kicking on your side but takes a little more concentration. You alternate between taking 3 strokes of freestyle, and doing 9 kicks rolled up on your side (same position as kicking on your side). The point of this drill is to get somebody to disassociate taking strokes and staying at the surface. In other words, some people forget that they’ll continue to float at the surface even when not taking strokes. The drill also challenges your body position by making you remain as streamline as possible between the 3 strokes because the 9 kicks are slower. In other words, you build up speed during the 3 strokes and if your body position is good, you will maintain more of that speed during your 9 kicks than if your body position is poor. Lastly, I think it’s a good coordination drill because you have to be able to bring your stroke cycles to a stand-still, and then get going again without any excess or unnecessary movements.