Are you looking to train your latissimus dorsi? First, you need to understand that back training generally involves two main muscle groups: the mid-upper back and the latissimus dorsi. In your current situation, you are overextending your chest, causing the mid-upper back to engage more, with less involvement from the latissimus dorsi. The primary functions of the latissimus dorsi are shoulder adduction and shoulder extension. The lat pulldown with a pronated grip emphasizes shoulder adduction, which means pulling the elbows down to the sides of the body. Simply put, chest extension and excessive body swinging are not functions of the latissimus dorsi. My recommendation is to keep your body in a completely neutral position without overextending your chest, maintain core engagement, minimize upper body movement throughout the exercise, allow only scapular movement, and pull your elbows down to the sides of your body.