Tearing by oneself or by another

I have seen that the custom of the Chevra Kadisha (Jewish burial society) is to begin tearing the mourner's garment with a knife, and the mourner finishes the tearing. Is it obligatory to do so?

In the "Birkei Yosef," it is written that it is preferable for another person, whom the mourner is embarrassed before, to perform the tearing, thereby making the mourner feel his mourning more intensely. However, in the "Aruch HaShulchan," it is written that it is preferable for the mourner to tear by himself (perhaps because the commandment is more fulfilled by oneself than by a messenger, or because this way he suffers more, or because he holds that the reason for tearing is to have something to ease his sorrow, and by himself, he eases it more).

Therefore, the custom is that a person from the Chevra Kadisha begins the tearing with a knife, and the mourner himself continues the tearing, for his father and mother until the heart, and for other relatives, he tears a handbreadth by hand. (A handbreadth is required because tearing less than a handbreadth is not considered tearing.)

The "Birkei Yosef" in section 12 in the name of the Mahar"i Molcho. Brought in "Pitchei Teshuva" section 1. "Aruch HaShulchan," Siman 340, paragraph 2, and these are his words: "Even for other deceased, where it is permitted with a knife, nevertheless, he must tear by himself and not through others, as this is implied from the language of the Gemara and the decisors."