Unless you have exceptional hand-eye coordination and arm strength, it’s extremely difficult to strike the ball solidly without involving your lower body.
By leading the downswing with the lower body, you utilize the strength in your hips while pulling the upper body – and the club – naturally into the proper positions. You’ll not only cure that over-the-top move, you’ll develop the coveted inside-out swing path – key to hitting draws and maximizing power.
Of course, it takes work to ingrain a lower-body-first downswing. The best practice method is the classic “pause-at-the-top” drill demonstrated in this video.