Rav Eliyahu Bakshi-Doron, former Sephardic Chief Rabbi of the State of Israel, in his work Binyan Ab (vol. 3, Siman 45), addresses an interesting question relevant to the large number of Jews from the Former Soviet Union who have recently immigrated to Israel and the U.S. Many of these Jews did not undergo Berit Mila (circumcision) in the FSU, and choose to have a Berit Mila at an adult age – a phenomenon which is certainly an extraordinary Kiddush Hashem (glorification of God’s Name). It is customary at these circumcision rituals for people to assemble to witness the Misva, and often a person is designated as the "Sandak." Under normal circumstances, of course, when an infant is circumcised, the Sandak is the individual given the honor of holding the baby on his lap during the Berit. When an adult undergoes circumcision, he lies on the table, and obviously does not need a Sandak. Nevertheless, there is a custom to assign a "Sandak" who stands behind the patient and holds his head. The question arises as to whether this indeed constitutes "Sandaka’ut" in the strict sense of the term.
The practical ramification of this question is whether or not Yehi Shem (no Tahanun) is recited in place of the regular Tahanunim at a Minyan where such a "Sandak" prays on that day. Normally, if a Sandak is present at a Minyan, that Minyan would recite Yehi Shem (no Tahanun) in honor of the Sandak’s festive occasion. The question thus arises as to whether this applies even to a "Sandak" who simply holds the head of an adult who undergoes circumcision.
Rav Bakshi-Doron cites the Hatam Sofer (Rabbi Moshe Sofer of Pressburg, 1762-1839) as giving two explanations for the purpose of the Sandak. Firstly, the child undergoing a Berit Mila is compared to a sacrifice, and thus the Sandak’s lap serves as the "altar" upon which the "sacrifice" is offered. Secondly, the Sandak participates in the Misva by enabling the Mohel to perform the circumcision more easily. The Sandak traditionally holds down the infant’s legs, which allows the Mohel to concentrate solely on the act of circumcision without having to be concerned about the child’s legs. By assisting the Mohel, the Sandak is able to take part in the great Misva of Berit Mila.
Needless to say, as Rav Bakshi-Doron notes, one who holds the head of an adult undergoing circumcision performs neither of these functions. He does not serve as an altar, because he does not hold the patient the way the altar holds a sacrifice, and he does not assist the Mohel in any way. If anything, his presence might be distracting for the Mohel. Therefore, Rav Bakshi-Doron rules, one who holds the head of an adult undergoing Berit Mila is not formally considered a "Sandak," and, as such, his presence in a Minyan does not warrant the recitation of Yehi Shem. Furthermore, such a person does not earn the benefits associated with Sandaka’ut, such as the reward of wealth and atonement for certain sins.
Incidentally, Rav Bakshi-Doron adds in this context that at an ordinary Berit Mila, it is proper for the Mohel to allow the Sandak to assist by holding down the child’s legs. Some Mohalim tightly wrap the baby’s legs, such that the Sandak simply holds the child and does not participate in any other way. As mentioned earlier, one of the purposes served by the Sandak is to assist the Mohel. It is therefore proper for the Mohel not to wrap the child’s legs, and to instead allow the Sandak to earn merit by helping the Mohel and thereby take part in the Misva.
Summary: It is customary during the Berit Mila of an adult for somebody to be assigned the "Sandak" and hold the patient’s head during the procedure. While this may be an admirable practice, the individual holding the head does not have the formal status of a "Sandak," and therefore his presence in a Minyan that day does not warrant the recitation of Yehi Shem in place of the regular Tahanunim.