Watch the fallMost beginners, if not all, let their hands fall too deep into the water when they first start to get the hang of freestyle. Whereas they should be reaching out and rotating slightly, so that they can extend their arm as far as they can. This drill is designed to fix that. It give the swimmer a chance to see what their arms are doing, so that they can get them to do what they should be doing.
Just to be clear this is not a drill for someone who has just learn the freestyle stroke. This is for those who are starting to do laps and are reasonably comfortable with their breathing. In other words not the complete beginner but someone who is beginning to develop their strokes.
Or its for the advanced swimmer who needs a refresher in reducing the fall in their freestyle arm.
It is important that as far as possible you do this drill without breathing. Particularly if you are relatively new to swimming. The last thing you need is to fix you arm fall and muck up your breathing.
Pay special attention to gradually moving your head back into it's correct position and concentration on how your arms Feel when they are in the correct position. There is no point to this drill if you can't maintain the fix after you go back to normal strokes, all because you have forgotten how the correct state feels.