Jin Choi is common to all Wing Chun systems but found in various places. Jin choi is found in the Yip Man System as a Pre Pole Drill. Its practised in the Sai Ping Dai Mah with stepping with punching. Obviously in the Yip Man system this is considered extremly advanced training as one doesnt practise it until the end of the system, when the Look Dim Boon Gwun is learned.
In the Yuen Kay San System, Jin Choi is one of the first things learned. Sum Num passed down that both of his sifu's, Yuen Kay San and Cheung Bo had this method. Sum Num Taught something called the Sup Yee Sik or the 12 seperate forms. The first 3 related the begining punching methods which include Pian San Choi, Duk Long Choi, and Jin Choi.
This short form places focus on Body power with focus on the waist and stepping uniting to power the punch.