Good Happens with a Connected Power Swing.
Swinging Bat Around Body Slows Bat Speed
For most young developing hitters, they learn to swing the bat around their body in a way that actually reduces bat speed because the upper body gets essentially disconnected from the lower body. This typically slows the turn of the hips and body, which will likely reduce bat speed.
Improve Bat Speed without Lifting Weights
To maximize bat speed, (just through the use of good mechanics and without ever having to lift a single weight), a hitter needs to learn how to connect his upper & lower body. When done correctly, this will: 1) improve linear drive of the lower half, 2) increase rotational torque (turn speed) of the hips and body, and 3) keep his hands inside the ball. All of these factors should immediately improve bat speed.