Energy cannot be destroyed. Hence, when doing push hands we have to listen to where this energy is going and provide an outlet for it. Once this energy is neutralized and no longer threatens our balance we can channel it back to the opponent to overcome him. This is what is meant by borrowing strength.
The key question is how do we create this outlet. The commonly seen response is to turn the body or sit back. This is not wrong. The problem lies in the sometimes excessive turning or sitting back that is disproportional to the energy received. This is why such training cannot be transferred to the use of fast striking exchanges.
To address this we use the 5-count mechanism which allows for a much faster response due to a more subtle chain of movement. The shorter response time is hence transferable to defending against strikes which calls for a much faster reaction and response time.
Interestingly, my student found that the 5-count mechanism can also be used to chop wood when he had to do so as part of daily chores whilst staying in a monastery recently. The 5-count is a mechanism that allows the body to generate a lot of power through the body whilst showing off minimal external movements. This is a closer approximation to what an internal art should be rather than the excessive huge movements that is marketed as internal.