Why do we hide our thumbs when we recite the Bracha on fire during Havdalah?

In Shulchan Aruch 298;3 it’s written to look at ones finger nails when saying the Bracha of Meorei Haish during Havdala, and the Rem"a adds to bend the fingers into the palm to see them together at ones. (the reason is in Tikunei Zohar in Hakdama and in Pshat – Mishna Brura ibid 9) The Igrot Moshe (OC5; 20) comments that this is a custom of the Tanaim (some 2000 years ago…) I couldn't find mentioned anywhere to hide the thumb, on the contrary, the Midrash (Shimoni Tehilim 723) says that it's the thumb that has to be used for Havdala.