Regarding someone who finishes the Shemoneh Esrei while a person remains standing behind him in prayer, the law is as follows: if he is farther than four amot plus another three steps away from him, so that if he were to take three steps backwards he would not enter into the other’s four amot, he may step back. If he is closer, he may not take steps back until the one reciting the Amidah behind him finishes his prayer. Even if the person behind him began praying late and extends his prayer, he is forbidden from stepping into the other’s four amot. As we have learned, the Acharonim disagree concerning the parameters of the prohibition.
According to the Magen Avraham, even if the person reciting the Amidah is not standing directly behind him, as long as by taking three steps back he will step into the radius of the four amot in front of him, he must wait until the person behind him finishes his prayer. According to the Eliyah Rabbah, it is only forbidden to step backwards in front of the person praying if he is standing directly in front of him. But if the person praying is not exactly in front of him, he is permitted to take three steps back. L’chatchilah, it is good to practice like the Magen Avraham, though in times of need one may be lenient like the opinion of the Eliyah Rabbah (Mishnah Berurah 102:18-19). Even in a situation in which the person praying is directly behind him, in extenuating circumstances he may step backwards diagonally, for there are those who explain that according to the Eliyah Rabbah, as long as his steps do not bring him closer to the person praying, he is permitted to step back (brought in the name of the Chazon Ish in the book Four Amot of Prayer, p. 50 and 363).
If between him and the person reciting the Amidah stands a person who already finished his prayer, he is permitted to step backwards, since the one who already concluded his prayer constitutes a divider between them, even if the one dividing did not take three steps back yet.
In times of need it is also permitted to be lenient when there is a partition that is at least ten tefachim (80 cm; 2.62 feet) high and at least four tefachim (32 cm; 12.598 inches) wide between him and the person reciting the Amidah. Those who wish to be lenient are permitted to regard the big permanent benches in the synagogues as a partition, since their height is at least ten tefachim.
Regarding one who cannot step backwards because of the person reciting the Amidah behind him, although he has not yet separated from the Amidah, he is permitted to answer Amen and Baruch Hu u’varuch Shemo and to recite all the prayers. If the congregation reaches Tachanun, he is permitted to sit and recite it, on condition that he does not sit directly in front of the other’s face. After that, he stands again in his place and when the person praying behind him concludes his prayer, he can take three steps backwards and say Oseh shalom (Mishnah Berurah 122:4 and see 104:9; also see earlier in this chapter, halachah 13).