The use the same profile as the Lyman M-die, which is now also used by Redding and RCBS
Rather than leave a flared mouth, it expands the mouth to form a "seat" with parallel sides. This shape holes the bullet more securely as it moves between stations and the parallel sides help the bullet align with the case as it enters the Seating die. While it certainly helps jacketed/plated bullets alignment, it is excellent in preventing "shaving" of coated bullets
