The first section of Chum Kiu teaches how to use turning and techniques at the same time, for example the Bong Sau and Wu Sau are performed whist turning and shifting the body weight from one leg to the other. This is teaching the practitioner to use the hips to develop power or Yiu Ma as it's called in Cantonese. The first section also introduces two way energy as seen when the Lan Sau arm Laps back and a straight punch is delivered.
This enables the practitioner to deliver more devastating blows with relative ease as the Laping arm is enabling the transfer of power across the body as the force can flow as one motion without interruption, with the addition of pulling your opponent off balance, the target will also be moving into the punch and so additional damage will be caused.
The key to doing this is to learn how to use all the muscles in your body in a short sequence ie. your hips and legs turn and start generating some power, which is then carried on my the shoulders and finally the arm.
If you miss time this, you end up just striking with your arm and not using the power of your whole body. The only way to develop this skill is through practice. Chum Kiu is a vitally important way to practice synchronizing the body's movements to work as one unit.