Yantras are much more powerful than a picture or even a statue of a deity and they are used to energize the latter. In Indian temples, the Sacred Inner Sanctums that house the main deity, the Yantra associated with that deity is placed under the base of the statue to initiate, energize and sanctify the space.
Yantras are Visual Tools
The geometrical forms of the Yantra activate the right hemisphere, which is visual and nonverbal. The diagrams are essentially thought forms representing divinities or cosmic powers which exert their influence by means of sound vibrations.
They can be illustrated on paper as mandalas or carved onto stone etc; however, the most effective substance to carry a Yantra is copper. Copper is a well known super conductor for electricity and cosmic energies alike. It is believed that constantly concentrating on the representation helps to build fortune (positive energies therefore positive potentials), as planets and deities have their peculiar electro-magnetic field which governs basic emotions and karma.
Therefore Yantras are visual tools that serve in meditation either as centering devices or as symbols of the energy pattern of a deity. When a yantra is to be used for worship and the energy is invoked in it, it becomes a symbolic representative of the deity. It actually becomes the deity when you can abandon your analytical, critical attitude and the energy circulates in higher centers.
Meanings of Some Shapes & Symbols in Yantras
Shapes and patterns commonly embedded in yantra include squares, triangles, circles and floral patterns. It may also include more complex and detailed symbols: