For a long while I thought that Qi is just a kind of force, and non-living. It must be wrong. Qi should be equavalent to spiritual being around us. In Tai Chi Chuan, Qi is the master and physical body obeys to Qi. It means that the physical body obeys to spiritual being, from outside.
The moment we practice Tai Chi Chuan, all parts of our body should be as balanced as possible. This is the condition for Qi to come to us. The slower is the movement, the easier for us to get our body to become balance.
How would the Qi becomes stronger? By attracting more spiritual beings to attach to us.
In the case of Wing Chun, the Qi will come by if we can manipulate our breath. To balance the breath in and breath out. The longer the interval the better. Especially during small thought practice.
Normally we associate advancing forward with initiating an attack, and retreating as a defence. It need not be always true.
When using 'thrusting finger', the thrusting motion could be used as a blocking, while we move forward.
When using 'sticky leg', the backward motion could be used together with the hooking of opponents leg, hence straightening his knee, preparing for another stepping onto the knee to break it.
Hence, there is attack & defence while moving forward. There is attack & defence while moving backward, so to speak, in Wing Chun.