A method of direct introduction the understanding of sunyata, of samsara and that the two are inseparable. Mahāmudrā literally means "great seal" or "great symbol." It "is a multivalent term of great importance in later Indian Buddhism and Tibetan Buddhism" which "also occurs occasionally in Hindu and East Asian Buddhist esotericism."
The name refers to a body of teachings representing the culmination of all the practices of the Sarma schools of Tibetan Buddhism, who believe it to be the quintessential message of all of their sacred texts. The mudra portion denotes that in an adept's experience of reality, each phenomenon appears vividly, and the maha portion refers to the fact that it is beyond concept, imagination, and projection.
Example video:
Example via www.ramdass.org: Mahamudra: The Moonlight – Quintessence of Mind and Meditation
Example via www.mindpodnetwork.com: The Teacher by Chogyam Trungpa