In regard to saying words of Torah without a head covering, the Shu”t Maharshal (72) writes, although there is no prohibition, and it is not even a middas chasidus [pious act] to refrain if one doesn’t mention Hashem’s name, a talmid chocham should refrain as people will learn from him and will come to make light of the issue. Practically, the Mishnah Berurah (2:12) rules, that it is forbidden to learn without a head covering.
If one isn’t speaking words of Torah himself, and is merely listening to others, it would seem that one should still refrain. The Pri Megodim (M.Z. 183:3) cites the Darkei Moshe who says, that if one is not wearing a head covering, it is permissible for him to listen to Hashem’s name being cited and one can be yoitsa a berachah using shom’ea k’oneh [listening is as if the listener himself spoke]. However, he then cites a Bach which says it is forbidden. In the end, the Pri Megodim sides with the Bach due to the rule of shom’ea k’oneh.
However, the Toras Chaim (91:1) writes, that even the Bach would agree, that if the listener places his hand on his head, it is considered enough of a head covering. Although, the Shulchan Aruch (91:4) rules, that when it comes to mentioning a name of Hashem, covering one’s head with his hand isn’t enough, regarding shom’ea k’oneh it’s enough, especially as the Darkei Moshe is lenient without a covering altogether. Additionally, the Mishnah Berurah (2:12) writes, under pressing circumstances one can be lenient and cover his head with his hands, even to make a berachah himself. Practically, if one is by the beach with some friends with his head uncovered, and he hears them talking Torah, he can listen in if he covers his head with his hand. By Torah there is no shome’a k’oneh. (See also Shu”t Divrei Yisroel 1:17 who writes, that under pressing circumstances one can be lenient and learn without a head covering, as long as one does it in his head, without looking into a sefer).
However, if one who isn’t wearing a head covering wants to speak in Torah himself, since the Mishnah Berurah cites a machlokes if in such a case covering with one’s hand is enough, and even those who are lenient are only lenient under pressing circumstances, he should ask his friend to place his hand over his head. Even if one’s own hand isn’t good enough, that’s because the hand and body are one, however, using someone else’s hand is not considered one and is considered a good covering (see Shulchan Aruch 91:4).
