I found a website with some exerpts from a Chinese wushu magazine article in which Yang Zhenduo answers several questions, including a question about “chousijin.” Note that the term was used by the questioner, and not introduced by Yang. He calls it “a kind of description of taijiquan’s neijin.” He mentions the phrase “yun jin ru chousi” (move jin as though drawing silk) in his answer. http://www.21bowu.com/zhonghuawushu/zhonghuawushu_2002_2/mjxx.htm
Here’s my quick translation of his answer on “chousijin.”
Taijiquan’s drawing-silk jin is a kind of description of taijiquan’s neijin. Because the movements of Yang style taijiquan are slow and gentle (huanman rouhe), they resemble the emiting of silk from a silkworm—slowly and gently coming forth, the jin/li is continuous and unbroken, the speed is even (junyun), without any phenomenon of suddenly going more quickly or more slowly. When doing taijiquan, you must pay attention that the rate of speed is evenly distributed (junyun), firm and steady, and you must pay attention that you fangsong. When you achieve true fangsong, along with and in integration with the content of the other requirements of taijiquan, threaded from joint-to-joint, this naturally produces neijin. When the neijin comes out evenly and gently (junyun xuxu er chu), not hurried or slow, it then produces the result of “moving jin as though drawing silk” (yun jin ru chousi).