6-3-6 drill is a progression of the Kick on Side and 6-1-6 drills. As with both of those drills, it helps to establish good alignment and a good catch position. By introducing three strokes every six kicks, the swimmer will find it more challenging to focus on achieving their optimal alignment and arm position.
Perform with fins and try to only breathe immediately after the three strokes. The rhythm is therefore kick for six - stroke - stroke - stroke - breathe - kick for six. Some swimmers may find this breathing pattern challenging, in which case, add an extra breath during the strokes.