Liber O's illustration of it is the photograph “The Silent Watcher”
See Illustration in Vol. I. No. 1, “The Silent Watcher.”
Rituals · sign
The foot drawn back and a forefinger laid on the lips: the gesture that closes a vibration and, in the Golden Dawn's papers, keeps what was sent out from returning.
Liber O calls it “the characteristic position of the God Harpocrates”.
Each step gives the action in plain language, the spoken words with their Hebrew, and the corresponding line from Liber O in The Equinox I:2 (1909). The drawings follow those instructions.
III 4 (e)
Draw the left foot back, and lay the right forefinger upon the lips.
(e) Then withdraw the left foot, and place the right forefinger upon the lips, so that you are in the characteristic position of the God Harpocrates.
See Illustration in Vol. I. No. 1, “The Silent Watcher.”
Then in the sign of the Enterer doth he direct the whole current of his will upon the sigil. Thus he continueth until such time as he shall perceive his will-power to be weakening, when he protects himself from the reflex of the current by the sign of silence, and then drops his hands.
From the Golden Dawn's instruction for evocation, cited here for the two signs only. The operation itself is not reproduced.
The edition is in copyright and is cited, not quoted.