Nick Bollettieri, who helped develop many top-ranked players, walks everyone through his flashlight drill.
“Turn those hips and shoulders, show me the butt of the racquet," he says.
His three key points are: hips turn, shoulders turn and extend the racquet out to the target. Your racquet handle is the flashlight, and you want your opponent to see the light for as long as possible.
The secret to getting deep shot penetration is not closing your body too soon, and turning off that "flashlight."