https://youtu.be/r82Mj0Nptz8
Here is some great slow motion footage of Nick Kyrgios practicing forehands.
Specifically, I would like to focus on the role of the left foot and stepping in—or lack thereof.
I believe that players lift their left foot to play heavy—to slightly increase the angle of approach of the racquet to the ball. Lifting the left foot slightly tilts the shoulders backward and causes the racquet pathway to the ball to rise more steeply. This helps the player kick the ball up with more spin and shape.