Kiddushin 22b - Acquiring a Shifcha with Bi'ah

The gemara in its discussion about acquiring a shifcha with hagba'ha and chazaka discusses whether you can acquire her with bi'ah. The suggestion would be that through bi'ah she essentially is carrying him, and should qualify as chazaka. However, the gemara starts making distinctions between an eved lifting his master where the eved does not receive hana'ah and having bi'ah with a shifcha where she receives hana'ah. At first glance, this seems to be an irrelevant distinction - who cares whether there is benefit or not? The bottom line is that she is "carrying him"!? However, after thinking about it a little more the gemara seems to be defining the type of work that would qualify as chazaka. Chazaka by an eved means to demonstrate that he is subjugated to the will of the master and must work for him. The rishonim have a discussion whether it is sufficient to do a work that does not directly involve the master such as sewing his clothes or preparing a bath for him. But all agree that the concept is to demonstrate servitude. When the eved lifts the master he is showing that he is obligated to carry the master so that the master won't have to walk. This must be done in a way where the eved receives no physical pleasure from what he is doing, because if he does then he is not demonstrating servitude. It is for this reason that the gemara understands that if the shifcha would be receiving pleasure from the bi'ah thereby possibly doing it for her own purposes, it would not qualify as a hagba'ha, since hagba'ah is not the kinyan, rather the kinyan is to show servitude.