Why didn't the Han Chinese shave their heads before? Why do they shave their heads now?

Before the Manchus entered the country, they asked the conquered or surrendered Han Chinese to change their hair styles in order to make it easier to recognize what was right and wrong.

Manchu men shaved the front of the skull, sideburns of all the hair, only the back of the skull to leave hair, braided into a long plait down, this increase in the scalp area of the hairstyle helps to wear helmets and hats in the war, the scalp area increased friction increased, can reduce the war helmet or cap crooked or fall off the chances of fighting to increase the safety and to avoid being accidentally killed by friendly forces.

The reason why the Manchu rulers implemented the "hair shaving order" was:

The shaving of hair as an important physical sign of whether the Han Chinese accepted the rule of the Qing Dynasty.

The Manchus were an ethnic minority, and suddenly they felt overwhelmed by the prospect of ruling a large, highly cultured and populous people.

Fear of Han resistance.

Establishing Manchu dominance ensured that the Manchus would not be assimilated by the Han.

The Manchu rulers hoped that by shaving their hair and changing their clothes, they would combat and destroy the national spirit of the Han, especially the upper classes.