If you find some previous tai-chi's methods that I talked about incomprehensible in terms of combat relevancy, one will truly be puzzled by this method called Kao (靠 - Leaning on). What is Kao or Leaning on? A prominent lineage Wu-style master teaching in US demonstrated the combat power of Kua through bumping people out using any part of his body (most notably, as in the classics, using his shoulder-joint). This technique has even been mystified by some practitioners as being able to bump their opponent out merely by the "feel" of one's skin without even actually seeing one's opponent (in "demonstrations", its like blind-folding the master's eyes). Why or when do one needs to bump one's opponent out (seemingly unable to do him much harm with such a bump!)? Instead of striking him with an elbow?
In the practice of form-pushing-hands Big Luo (大捋), shoulder Kao is used as an oblique attack after one is being counter-attack by a Cai (your opponent tries to unbalance you after locking your elbow). In combat mentality, it is like throwing your body against your opponent in a disparate attempt to balance oneself and to bump both to the ground with one's bumping force! Like some Taoist meditation techniques, this technique of Kao does look a little silly at first glance!
The gist of Kao is to throw your opponent to the ground by bumping him in close counter without oneself falling the ground at the same time. It is used in situations when one is at an disadvantaged or vulnerable position. Kao is a surprise attack. It requires a high level of training in Muscles-as-one, a firmly rooted feet together with a truly shocking bumping power! Truly an esoteric attack technique, and, as with anything that can be called esoteric, having a small window of opportunity to be executed successfully in real combat even by proficient practitioners. A combat technique of last resort!
The closest combat situation is using one's body weight to break the structure of your weaker opponent who wants to throw you (like when he tries to use a hip throw and you simply weighs him down!)
The final question: How to use chi to execute Kao? The answer: Like you relax totally when falling into your bed after a tired day.
The above answer may look silly but is actually not. If you have experience taking care of the elderly or an invalid, you will know how heavy (and powerful if such heaviness can be managed) it will be for a person to completely reflex himself.