In the worldview of the Tantra sadhaka the whole world consists of Shakti, every material and every moment has its own unique Shakti. A good knowledge of this allows the seeker to use different and exact materials, gathered at specific times using the knowledge of Tantric jyotisha, and combined by the application of powerful mantra-s to cause some effect that is universally visible to all.
When an Amavashya (new moon) coincides with a Tuesday, such a combination is extremely potent for creation of a kavacha with the power of Devi Durga. A kavacha is like as astral armour that protects the individual from all sorts of attacks that s/he may face originating from either the visible or invisible worlds around.
Photo: Durga at Galaganatheshwara temple from Karnataka. 12th century murti consecrated during the reign of the Western Chalukya empire.