When I first started learning internal systems from a legitimate master of internal systems, one of the first and most significant things taught was the Six Harmonies.
A major difference in internal systems, and external systems is the emphasis on “Everything starts together, and everything stops together”. It is a common thing constantly repeated with most internal martial arts. This includes Liu Ho Pa Fa, Hsing-I, Bagua, most styles of Taiji, etc.. Once you’ve learned proper structure/alignment you need to learn how to move.
There are six External Harmonies, and six Internal Harmonies.
External Six Harmonies:
The shoulder and hip move together. They start and stop together. the alignment is right shoulder and left hip, left shoulder and right hip.
The same relationships exist between the elbow and knee, as the shoulder and hip. The elbows should never extend past the knees.
The same relationships exist between the hands and feet, as the shoulder and hip. If any of these six are moving, all should be moving. If one is not moving, none should be moving. Keep in mind the proper relationships.
Internal Six Harmonies:
If you want to kill the mind must turn cold.
Chi is directed anywhere in the body by the mind.
The muscles and bones join together to produce power.
These principles work with other principles to produce greater power. Each teacher/system may manifest the six harmonies with their own flavor. You must have a qualified teacher to truly learn them.