This is a patented technology used in YONEX badminton rackets. It is a T-shaped part that is installed at the joint between the middle pole and the racket frame. The full name is "T-JOINT BUILT IN". The purpose is to improve Racket body stability and torsion resistance, etc.
However, although fake YY products (including all so-called factory products, original products, etc.) claim to be equipped with T-heads, it has no practical significance.