Some rumors about China’s stance in 71 conflicts need to be clarified.
1) Chinese military doctrine prevents it from sending troops abroad, until Chinese territorial is under direct threat of hostile forces, and/or also under the request of the neighboring friendly country. This is exemplified in Korea war, as well as in 62 Sino-India conflicts and Viet Nam conflicts. In latter two cases, only “direct threat” applies.
2) Then Chinese premier Zhou Enlai commented to Nixon and Kissinger that if India’s military involved in then East Pakistan’s conflicts, China “will not sit still.” (see the declassified document of US Department of State). Americans, by their own, interpreted this as China will send troops. (As a matter of fact, Nixon suggested China sending troops and US would support militarily as well. Also see the same file) This is a deadly wrong conjecture. Some other guesses such as China is going to parachute troops in BD is even more ridiculous. China was then in a chaos, and PLA functioned as a stabilizing factor in China. The main reason that China would not send troops in BD, however, is that it is a complete violation of Chinese military doctrine: Chinese territory was not threatened by BD conflicts. But, the Chinese did not sit still, it amassed troops along Sino-India board, and annihilated couple of Indian military posts established after Chinese troops withdrawal to their own territory in 62. China also issued an ultimatum-like waning to India. What China worried gravely at that chaotic time was that India had amassed its forces along West Pakistan board, and might well invade West Pakistan, and that had to be stopped. And it was stopped, as India obviously felt incompetent to fight in two fronts.
In conclusion, it is very ridiculous to imagine that China would send troops to BD, but it is true that China exerted huge pressure on Sino-India board to force India to abandon any attempt/plan to invade westward and to relieve pressure on West Pakistan