BAI SI (literally 'Bow to Teacher') is the traditional ceremony of formal acceptance of a student by the Sifu.
The definition of the term "Sifu" ('Father-Teacher' or adopted father) goes beyond that of instructor, following the old Confucian principle of Filial Piety. Thus the formal acceptance of the student by the Sifu is actually an adoption ceremony. When an individual chooses and joins a martial arts school, there instantly starts a Sifu-student relationship with the Master of the school. You are considered one of the regular students and nothing is asked of you, nothing is demanded of you, as you are just there to learn kung fu. This individual typically sees the martial arts school as a hobby and as such their attendance may be sporadic. As a result, the progression of the individual may also be sporadic and unpredictable.
The longer a student trains the more likely they will no longer identify their kung fu training as just a hobby. That individual now wants to get a deeper understanding of the system. To some this may mean entering an instructors program where they can obtain a certification to teach the art and potentially make a profession out of Kung Fu and being a professional martial artist. There is another level of student who truly identifies with the system and wishes to make a lifetime commitment to the system and the family. Bai Si, also known as discipleship, is the next step the individual would take to be accepted into the Kung Fu family.
The prospective disciple kneels before the Sifu, bows, and then serves the Sifu tea. ]The sifu taking and drinking the tea signifies the student's acceptance into the family as a disciple. Old Chinese residential architecture give a large gate into a large courtyard. On the other end of the courtyard are the inner rooms. The two Bai Si levels are based on this; the Yap Mun/In The Door is the first one, and signifies the disciple has been let "in the gate" and into the courtyard. The Yap Sut/In The Room level is the higher one, and signifies one has traversed the courtyard and is even closer to the Sifu. After being accepted into the kung fu family, one is thought to be closer to the Master of the system then his/her own family/siblings. You are now in the eye of the system. Anything thing you do and don’t do will have a very important meaning, not only to the school, but to the Master and the system.
Bai Si, as in the eastern thinking, is always thought of as the actual time when you start to learn kung fu. Beforehand, there may be corrections here and there, but now, you get the in depth understanding of the system. As the training continues, the proper etiquette is to not ask for more material. The Sifu will know your progression and your period of advancement. Bai Si, as explained, is a very important. With the proper guidance from the Sifu, the disciples will carry on the system to the following generations - a step and part in the kung fu family.