So what is Bai Shi ? In the context of Chinese martial arts it is a ceremony with ritual elements conducted by a master in which one or more students "Enter the Door" and become disciples.

After the conditions of Bai Shi have been read or told to the students, they agree to accept them and the ceremony begins. Normally this would be at the master's home or studio where there would be a portrait of the founder of the style. Usually, but not always there is a fee paid by the student traditionally in a red packet as red is a propitious colour and it is considered indelicate to display money openly. The master then places an offering of fruit in front of the portrait of the founder and lights a ritual number of incense sticks which he gives to the student who then kneels down before a portrait of the founder of the style and gives the koutou (literally knocks the head) three times to show his respect to the founder's memory. The student then faces the master and again gives the koutou. The incense is then placed in an incense burner in front of the founder's portrait. The ceremony is over; the student has entered the door.

So what firstly are the implications of the ceremony ? The student by undergoing Bai Shi has made a commitment to the school, to the founder, to his kung fu brothers and sisters as well as to his master. The master recognises this commitment by allowing the student to enter the door and in turn makes a commitment to give the student the true transmission of the art and to start to give him inside the door training such as Nei Kung. The student can now be referred to as Men Ren, literally "door person" and is no longer a mere student.

References:

www.taichichuan.co.uk/information/articles/bai_shi.html