The Ying Yang is the most fundamental and universal concept which forms the backbone blueprint of Kung Fu itself. However the amount of its applications and interpretations will vary between the different styles.
Ying Yang concepts apply to both indemnity and defense and can be looked upon from either a positional, structural or energetic standpoint. In essence it is how we simultaneously apply and interact all our elements of 'hard' and 'soft' to regain or to disrupt harmony.
In its simplest form a positional example would be setting up a fighting posture with one hand forward and one hand behind. From an energetic perspective one may visualize one arm pushing while the other is pulling.
As we look deeper into this concept of structure and energy, Ying Yang rotation can be executed in isolation to different parts of the body wherever at the hand and wrist or from the waist and hip position. Its universal nature allows it to function at various angulations whether on the horizontal or vertical planes.
In basic application we can demonstrate this for example in a case where both your arms are simultaneously and individually grabbed with force. Using the concept of Ying Yang, one can counter this simultaneously lifting one arm and pressing with the other arm (so that the arms move in opposite directions) while at the same time extending one hand and retracting the other one. If also at the same time we execute a rough latitude angulation of say 30 on the top hand and 210 degrees to the other respectively then if timed properly we should effectively dissipate most of the power from the initial grab.
The same strategy however can be reversed and used offensively to create an opening in our opponent's structure whilst at the same time distorting their energy to delay their reaction time. As in the previous example we apply a lift-press / push-pull with angulation along with explosive Ging power.
There exists many levels of understanding and applications of Ying Yang, but once we have a fundamental understanding of Ying Yang we can apply it strategically along our other Kung Fu concepts such as Centreline and Circular Angulation.