In a running FH, to initiate the backswing, you consciously bring the racquet "down" or under the ball. This will cause the elbow to pivot high as the racquet is taken back (like Sampras.)
This has big advantages when moving laterally. You don't need as much torso rotation to complete the backswing and forward swing. As a result, you can still swing through the ball with little restriction. Your stance will naturally be more closed.
Directory of Tennis Warehouse Clubs (courtesy of Mountain Ghost)