1) is the lever principle and the most used principle.

When a small force is applied at the end of the long arm, a large force can be obtained at the short arm at the other end of the fulcrum.

This definition is not the original meaning of "four two dial a catty", but: four two make a catty.

2) It is the principle of external force and the original meaning of "four Liang".

When the opponent punches, if there is a lateral external force to gently push the fist during boxing, the fist will change direction. This is: four or two thousand catties.

Another example: a bullet can penetrate a steel plate, which is considered as "a thousand pounds of force". However, the lateral three-level wind force is enough to change the trajectory (the bullet's travel route) so that the bullet can't hit the target. How much force can the third-level wind force act on the bullet? Just like the spring breeze, it is definitely less than four or two.

3) Four pairs form a thousand pounds of force. But not the lever principle.

A practical example is: when a car collapses into a pit, connect the car with a nearby tree with hemp rope and straighten it as much as possible. Then, pull horizontally in the middle of the hemp rope and you can pull the car out of the pit. This is actually the principle that a small resultant force produces a large component.