This drill is designed to focus on an area of the stroke directly under the head as the swimmer moves from the catch to the push phase. The drill is extremely useful if you have a tendancy to pull wide with your arm stroke or pull through with a straight arm. Scull the water directly under the head by keeping the elbows fixed in place, keep the palms facing back behind you... imagine you have drawn a smiley face on the palm of your hand and are showing it to someone behind you. Scull from just wider than your body's width into the centre. Don't tense up as you do this drill, try and stay relaxed and breathe smoothly.