The Rama (Rabbi Moshe Isserles,Poland 1525-1572) writes (Orah Haim 293:3) that the custom in the Arbit of Mosa’eh Shabbat-the Saturday night prayers is to stretch the Vehu Rahum and the Barechu. The rabbis say, that those who are careful with this custom will have no damage befall them in the coming week. The simple reason for this custom is that we do not want to rush the departure of the Shabbat, and on the contrary we want to add holiness to the mundane, so we stretch the Barechu.
Others suggest another reason, as to why we stretch the Barechu on Mosa’eh Shabbat. The souls of the sinners leave gehinnam for the Shabbat, only to return after the hazan says barechu on Mosa’eh Shabbat. So in order to give more time for their souls to remain out of gehinnam, we stretch the Barechu.
Summary: One should be careful to follow the custom brought down by the Rama, to stretch the Vehu Rahum and the barechu in the Arbit of Mosa’eh Shabbat, the Saturday night prayers.
