The Halachic authorities debate the question as to the amount of skin that needs to be removed when performing a Berit Mila. It is clear that Berit Mila requires removing the Orla (foreskin), which is the skin covering the Atara ("crown") – the upper part of the Eber (male organ). However, less skin covers the Atara when the Eber is in an erect state than in its ordinary state. The question thus becomes whether the Mohel must remove all the skin that covers the Atara normally, or only the skin that covers the Atara when the Eber is erect. The Shach (Rav Shabtai Ha’kohen, 1621-1662) advances the lenient view, that it suffices to remove the skin that covers the Atara when the Eber is erect, and a number of Poskim follow this position. The Shulhan Aruch, however, rules stringently in this regard, requiring the removal of all the skin the covers the Atara when the Eber is in its ordinary state.
Therefore, Sephardic Mohalim must be trained to remove all the foreskin that covers the Atara in the organ’s normal state, in accordance with the Shulhan Aruch’s stringent ruling.
Moreover, if a Berit is performed on Shabbat, and the Mohel removes only the skin that the Shach’s lenient position requires removing, he must go and remove the remaining skin required to be removed by the Shulhan Aruch. If he does not, then what he has done does not constitute a valid Berit Mila, and he is thus considered to have violated Shabbat.
After the removal of the foreskin, the Mohel is required to perform a procedure called "Peri’a," which means tearing the thin membrane over the Atara and pulling it down. The Shulhan Aruch rules explicitly (Yoreh De’a 264) that if one removed the foreskin but did not perform "Peri’a," then the circumcision is invalid, and the baby is considered uncircumcised. Therefore, on Shabbat, if the Mohel removed the foreskin but did not perform "Peri’a," he must go back and complete the process by performing "Peri’a," as otherwise he is considered to have violated Shabbat.
It sometimes happens that two Mohalim jointly perform the circumcision, with one removing the foreskin and the other performing "Peri’a." The question arises in this case as to whether the second Mohel must recite a Beracha before performing the "Peri’a," or if he is covered by the Beracha recited by the first Mohel before he removed the foreskin. The Machshireh Mila (Hacham Eliyahu Shama Ha’levi, Chief Rabbi of Aleppo, d. 1814) rules that if the first Mohel knew at the time he recited the Beracha that a second Mohel would be performing the "Peri’a," then since he had the second Mohel in mind, the second Mohel does not recite a Beracha before performing the "Peri’a." If, however, the first Mohel recited the Beracha assuming that he was going to complete the entire procedure, but then he was unable to perform the "Peri’a" – such as if his fingers could not clasp the thin membrane – requiring somebody else to step in, then the second person recites a Beracha before performing the "Peri’a." Since the first Mohel did not have the second individual in mind when reciting the Beracha, a new Beracha is required. In such a case, then, two Berachot would be recited over the same circumcision.
Summary: According to Sephardic tradition, a Mohel performing a Berit must remove all the skin that covers the "Atara" (the "crown," or tip, of the male organ) in its ordinary state, and not only the skin that covers it in its erect state. If a Mohel had planned to perform the entire circumcision, but after removing the foreskin was unable to perform the "Peri’a" (pulling down the thin membrane underneath the foreskin), requiring somebody else to come and perform the procedure, the second person recites a Beracha before performing the "Peri’a."