In the above video, taken in Chicago on the occasion of the publication of "Mnach Yoma," Rabbi Gettinger explains his views on Bein Hashemashsos.  His understanding of Bein Hashemashos was premised upon his view of how the Sages understood cosmology, which he described in "Elements of Talmudic Cosmology" (1989), published in a collection of essays in honor of the 60th birthday of Rabbi Yehuda Cooperman, founder of Michlalah-Jerusalem College.​

The topic of Bein Hashemashos is addressed primarily in Mesechta Shabbos.  However, Mesechta Pesachim contains an important discussion of "shkiah," and includes a critical comment of Rabbeinu Tam on a seeming contradiction between these two gemaras.  Rabbi Gettinger addressed Bein Hashemashos at length in a series of shiurim that can be found here (starting on Shabbos 34B).  Rabbi Gettinger's style of learning is nothing if not thorough, as can be gleaned from a brief interchange between him and a student concerning how they would approach learning Bein Hashemashos.  When they concluded the topic, 29 shiurim later, Rabbi Gettinger provided a brief summary of Rabbeinu Tam's seminal comment on "shkiah."

Bein Hashemashos (בין השמשות)

Rabbi Gettinger had a particular interest in Bein Hashemashos, and spent many years delving into this topic.  The crowning achievement of these studies was the publication of his magnum opus “Mnach Yoma,” a ground-breaking discourse on Rabbeinu Tam's opinion on Bein Hashemashos.  You may order a copy from Ezras Torah at this link.

Rabbi Emanuel Gettinger