Loose arm,shoulder, and grip, is the key.
Steve s
Steve s comment is right on!!! Daniel K post is right on also. In order to swing faster the arm, shoulder and grip must be relaxed. It's not about muscles in the arm, but muscle energy
from the legs and hips that transfer that energy through the weaker muscles of the arm and grip. The energy produced by the lower body mass must not be restricted by tight muscles
of the shoulder, arm, and grip. It's called "The Kinetic Chain". This chain is what produces racket head speed, not arm strength or wrist snap. It is the Kinetic Chain that creates energy,
power, and higher velocity of a racquet head contacting the ball.
An analogy would be a whip, the movement of the handle accelerates the tip of the whip at tremendous speed. Or think of a rolled up towel, wetted, and used as a cruel joke after
a gym shower. The tip of that towel hurts on contact because of the rapid acceleration of the towel tip trying to keep up with the handle. I know not the best examples, however,
much like the wheels on a car when making a turn. The inside wheels turn slower and the outside wheels have to turn faster to keep up.
So, keep the upper body small muscles relaxed and let the energy produced travel through them for more racquet head speed. There is a plus side to using this approach, it is
definitely easier on the arm. The clinching of the grip and arm muscles is what contributes to tennis & golfers elbow along with pronating while clinched.
Aloha