what is T-Join, T-Join is bulid inside the racket, it's made by metal, you have to use X-ray, then you can see it inside the Racket
Right side is with T-join inside.
T-join is good at power transmit. when you hit the ball, power transmit from your arm, then to the racket.. at this time , when the power transmition from racket stick to the "T point", will lose much power.. T-join would keep 30% power more here. that's why T-join normally better than without it.
PS: normally, T-join Racket with a stiff stick, because soft stick will lose more power.