Think of the baseball swing as a chain of events. Power is built at your body’s foundation and it’s transferred to other parts of the body before eventually reaching the baseball bat. Each movement and body part plays an important role in the transfer of energy. Inefficiency then comes when muscles are activated too early, too late, or not in the correct order.
A proper swing path gets into the hitting zone quickly and stays in that zone as long as possible. The longer the bat is cutting through the hitting zone the better chance you have to hit the ball more consistently. This "metric" is vital for achieving optimal launch angles and consist barrel contact. Notice that Evan Longoria has tilted his shoulders to turn at a 45 degree angle, this keeps the bat "behind" the ball for as long as possible.
The stretch and fire concept happens when most players throw a baseball. The lower body leads & the upper body stays back until the very last second. If you look at the good hitters throughout the history of baseball, they all keep their hands back, allow the lower body to lead, which creates torque and leads to more power. They have learned how to use stretch and fire and take a full swing at every pitch.