Touching the earth as Gautama did, to invoke the earth as witness to the truth of his words.
Bhumisparsha means touching the earth. It is more commonly known as the earth witness mudra. This mudra, formed with all five fingers of the right hand extended to touch the ground, symbolizes the Buddha's enlightenment under the bodhi tree, when he summoned the earth goddess, Sthavara, to bear witness to his attainment of enlightenment.
The right hand, placed upon the right knee in earth-pressing mudra, and complemented by the left hand-which is held flat in the lap in the dhyana mudra of meditation, symbolizes the union of method and wisdom, samsara and nirvana, and also the realizations of the conventional and ultimate truths. It is in this posture that Shakyamuni overcame the obstructions of Mara while meditating on Truth.
The second Dhyani Buddha, Akshobhya, is depicted in this mudra. He is believed to transform the delusion of anger into mirror-like wisdom. It is this metamorphosis that the Bhumisparsha mudra helps in bringing about.