Many golfers believe their first move in the downswing should be a body turn.  It may look that way when you watch Tour pros at normal speed.  But watch those players in slow motion. You'll see a lateral movement of the body in the downswing that happens a fraction before the turn: a sideways brace of the left leg.  This left-side brace "buys you time" to allow your arms to drop the club in the slot before you start your turn. 

The clubhead travels in three dimensions in the downswing: down, out and forward.  If you turn too soon your body invades the space where your arms should be. The club then goes out first, making its movement out, down and in.

Your teaching pro calls it over-the-top or outside-in.  You know it as a slice.

The Fix

I get my favorite image from my favorite breakfast: Cheerios. Place a single Cheerio down the target line, 2 inches in front of the ball and parallel to the outside edge of the ball.  In your downswing make sure you hit the ball, and then the Cheerio, to ensure you are hitting down and out.  If you struggle doing this, it's time to hit some half-swings (hands hip-high in your backswing ). Start the downswing by bracing your left foot. Then slot the club so that the shaft is aiming toward the ball or target line.  Focus on the clubhead hitting the ball and Cheerio.  When your divots are straight and the Cheerio is gone, you are on the right path to success.

The Feel

To start the downswing, your body should bump left while your arms drop. You should feel like you are "backing into the target" with your body, giving your arms a lane to drop those precious few feet. Then you can turn, as hard as you want.