Commentaire sur Berakhot 2:4

נדבך – a layer of stones of a building such as “a course of unused timber for each three courses of hewn stone” (Ezra 6:4), and even though he is fearful that he might fall, and would not be able to focus (“have spiritual intentionality” during his recitation of the Shema) , the Sages did not require him to come down, for the Recitation of the Shema only requires spiritual intentionality for the first verse only (“Hear O Israel, the LORD our God, the LORD is One”).

Introduction This mishnah deals with reciting the Shema in a place where having proper intention might be very difficult.

מה שאינן רשאין לעשות כן בתפילה – for Prayer (the Recitation of the Amidah) is [a plea for] mercy and requires spiritual intentionality. Therefore, one descends (from above) and prays (recites the Amidah).

Workers may recite [the Shema] on the top of a tree or the top of a scaffolding, that which they are not allowed to do in the case of the Tefillah. The mishnah describes a case of workers who go to work early in the morning and find themselves working in a tree or at the top of some scaffolding when the time to read the Shema comes. The mishnah allows these people to recite the Shema without forcing them to come down and stand on the ground. However, when it comes to the recitation of the Tefillah, which means prayer and refers to the Amidah prayer, they must come down. There are several reasons for this difference between the Shema and the Amidah. First of all, the Amidah is much longer and therefore will require more concentration. Second, the wording of the Shema was completely set by the Torah and there was no room for improvisations. While to fulfill one’s obligation one had to have the intent to perform a mitzvah, it would not have required a tremendous amount of focus. In contrast, the wording of the Amidah was more fluid and probably less familiar. Therefore, when it comes to the Amidah they have to get down from the tree and only then can they pray.