To understand the application of these postures you need to think in an unconventional way, that means not thinking in terms of blocking and punching. There are no blocks in tai chi instead it uses the techniques of following, joining and dissolving of force and at higher levels redirecting the energy back to the opponent. We need to learn to never resist but to listen to and follow the incoming force.
Slow Form is training for Pushing Hands and in turn Pushing Hands is training for Slow Form, they are both inseparable as each supports and gives understanding to the other. you can't understand Slow Form without Pushing Hands and in turn understanding in Pushing Hands changes and develops your Slow Form.
The Brush Knees are not block and punch even though they appear to be, there is no fixed response in tai chi application, instead they are teaching you to combine and follow. They are based on understanding the following rules:
Peng (warding), Lu (diverting), Ji (pressing), An (pushing), Tsai (plucking), Li (twisting), Zhou (elbowing), Kao (leaning) and the five steps, stepping forward, stepping backwards, turning left, turning right, and central balance. In Pushing Hands you use these techniques to develop: Ting (listening), Zou (leading by walking away), Nien (sticking), Hua (neutralising), Fa (attacking).