The Javelin drill begins with the kick on side drill, wearing fins, but this time with the leading hand wearing a Finis Freestyler Paddle.  Approximately half way through the length, switch into full freestyle, but only breathing away from the paddle.  So if the paddle is on your right hand, only breathe to the left.  

The aim of the drill is to correct an arm cross over, particularly when you take a breath (which is the most likely time for a cross over to occur).  The first part of the drill gives you the opportunity to find good alignment, without the complication of the stroking arm.  Draw your shoulder blades back and point your middle finger directly down the pool.  When you move into the freestyle stroke, the paddle will help keep this good alignment when you take your breath.  If your arm does cross over, the paddle will wobble, giving you the signal that you need to adjust your arm position.  Focus on keeping the lead arm aligned with your shoulder, pointing straight down the length of the pool, as you breathe.

Try this drill on both sides - you may find that one side feels harder than the other.