Our body has many pockets of energy that wants to go its own direction. This may be from previous training or habitual usage. In push hand these multiple energy does not work together, and it results in leak and blockage.
Immersing lumber in wood is similar to practicing foundation. The moves inprovides a structure and the energy must stay within it. The arm has its principles, waist can only turn, and kua can only open.
I asked him does this also mean to practice with no power. No, power has to be there but it has to be evened out. Students should not train via having no power route because it will be hard to get it back.
Once the energy always stay inside the structure then the body is trained and can move to the second stage of training.