This Pioneering Lineage "Vaishnava Tantric Bauls of Birbhum" Bengal use a yantra called the 'Gopi Yantra' as their Yantra 'instrument of focus' while others use a mandala the mandala (yantra) for this lineage is the Gopi Yantra. The Ektara was only brought into focus by this lineage. They have been using it for thousands of years a Vaishnava Tantrics. The difference between orthodox Vaishnava and the Vaishnava Bauls of Birbhum is that they are Tantrics. They are Vaishnava Tantrics but embody all 3 murti's Brahma, Vishnu, Shiva.
The Gopis are the lovers of Krishna, when Krishna would play his flute they would go mad with divine love and come running to Krishna. The sound of the Baul 'Gopi Yantra' can make one mad with divine desire of love, the Gopi Yantra is a mystical trance instrument. This yantra is infused with Shakti, Nabani's was super powerful and many wanted his Gopi Yantra.