One yantra is comprised of several geometrical figures like circles, triangles, horizontal and vertical lines, point, square, etc. These are carved on metallic plate from several dimensions and angles. It may look downward, upward, etc.
Concentrating on these symbols in yantras can also help in brining good karma, getting rid of the ill-effects of past karmas, etc. Each geometric element in the yantra represents five nature elements in the universe like air, water, fire, knowledge, etc. These symbols are inscribed on metal sheets with the help of mantras by which the symbols get the power to represent cosmic powers and divinity.
Similar way, chanting mantra after wearing the yantra on one’s body will also help the aspirant to evolve spiritually. It is a kind of esoteric concept which is not followed by everyone. While yantra is something physical and gross, mantra is subtle, still more powerful.
Scientifically, it is said that the geometrical shapes carved on the yantra is just an epitome possessed by the human brain. That is the reason why the mind is attracted towards certain symbols which has created impressions on the human mind in previous births. These yantras cannot be made anytime, but prepared during certain date and time, according to the procedures defined in Vedas.
Some people who believe in certain deities wear yantras with a belief that it protect them from bad misfortunes and bad luck that arise in the life because of past karma. Precisely, mantra and yantra are based on tantric principles to be used as tools to evolve spiritually.
Both mantra and yantra has the power to transform the mind as decided by the aspirant. Yantras are also used in black magic which can be harmful for the person if not used in proper ways.