The idol Conrad has in mind is a statue of the Buddha, for the posture Marlow adopts is that of Siddhārtha Gautama at the moment of his enlightenment.
In countless idols and paintings the Buddha is depicted with his arms dropped, specifically with one or more fingers of the right hand touching the earth – a gesture known in Buddhist tradition as the Bhumisparsha Mudrā (the earth-touching gesture). According to Buddhist legend, when the Buddha entered his most significant phase of meditation under the Bodhi Tree (the tree of enlightenment, represented here by the Nellie’s mizzen-mast), he was tempted by Mara, the god of darkness. At this point, the Buddha 'touched the ground with his hand, and the Earth itself spoke with a voice of thunder: 'I am his witness.' Conrad may well have heard the story of the Buddha’s enlightenment during his travels in the East.