A Moment with the Rebbe: At the famous farbrengen of Shmini 5712, the Rebbe encouraged the third stanza of Shalosh Tenuos. Facing the wall, the Rebbe swung his hand, as if to say: deeper, deeper.
Reb Yoel Kahn relates:
At the Shabbos Parshas Shemini farbrengen of 5712 (1952), the niggun “Shalosh Tenuos” was sung, which is comprised of stanzas from the Baal Shem Tov, the Maggid and the Alter Rebbe.
The niggun continued for a while. The Rebbe repeatedly asked that third stanza be sung, again and again – “This is the part that is relevant to us: the Alter Rebbe’s stanza, the new neshama…”
When the farbrengen ended, the Rebbe went to his room and returned shortly thereafter for mincha. After davening, the Rebbe began singing the niggun again, with an emphasis on the third stanza.
The Rebbe turned to the wall and leaned one hand on it. The other hand he closed but left the thumb open, and began making swinging motions, as if to say: deeper, deeper…
(Bedarkei Hachassidim, page 226)