The Chinese were forced to wear pigtails by whom and for how long and what superstition is involved in the cutting of the pigtail?

The pigtail was a mark of political enslavement to the Manchu dynasty, and westerners made fun of the Chinese hair style. The pigtail was abolished in China in 1911 when the Manchu dynasty was overthrown in favor of the Republic. Cutting off a pigtail was a terrible insult to the Chinese as they believed it would hurt them spiritually.

I also found this reference:

"Originally it was a sign of submision demanded by the first Manchu Emperor only one year after seizing the throne in 1645. Chinese men shaved the front of their head, then combed the back hair into braids. They used not only their own natural hair, but also horsehair or black silk. The Chinese came to see their braided pigtails a sign of dignity and manhood. For the Chinese, to pull on another's pigtails was a great insult."

So the custom would have lasted for 265 years.

Chinese boys and men until the Revolution were famous for wearing plaited pigtails. This was a fashion introduced by the Manchus in the 17th century. Originally it was a sign of submission demanded by the first Manchu Emperor only one year after seizing the throne in 1645. Chinese men shaved the front of their head, then combed the back hair into braids. They used not only their own natural hair, but also horsehair or black silk. The Chinese came to see their braided pigtails a sign of dignity and manhood. For the Chinese, to pull on another's pigtails was a great insult. The pigtails were abolished in 1911.

http://histclo.com/country/other/china/hair/chs-pig.html

The line of people answering this is very apt as the pigtails were also known as queues.

Prior to cutting them off, elderly Jingpo men would have a pigtail tied on top of their head covered with a black turban. Young people prefer white turbans.

Some of the men wore a kind of leather scullcap with a curtain behind covering the neck - the front of the cap was often decorated with teeth of wild animals.