The Chun Kui or again referred to as the second section is training how to use use the power / strength that has been built up in the first section. In Wing Chun Kung Fu, the strength and power are used half soft, half hard.
This is easily demonstrated when throwing a punch, your arm travels at great speed but the muscles are relaxed, this is the soft part. But just before you make contact with your opponent, your muscles in your arm tense up for a split second, this is the hard part.
This later develops into full delivery of the Kinetic Energy of your arm and body into the target, without compromising balance. In Chinese martial arts, good use of this on off energy is often referred to as Ging.