Vrtti literally means a whirlpool. It is a thought-wave in the lake of citta. Citta is the mind-stuff or mental substance. It takes various forms. These forms constitute vrttis. It gets transformed or modified (parinama). These transformations and modifications are the thought- waves, whirlpools or vrttis. If the citta thinks of a mango, the vrtti of a mango is formed in the lake of citta. This will subside and another vrtti will be formed when it thinks of milk (milk-vrtti). Countless vrttis are rising and subsiding in the ocean of citta. These vrttis cause restlessness of mind. Why do vrttis arise from citta? Because of samskaras (habits) and vasanas (desires). If you annihilate all desires, all vrttis will subside by themselves. When a vrtti subsides it leaves a definite impression in the subconscious mind. It is known as samskara or latent impression. The sum-total of all samskaras is known as karmasaya (receptacle of works). This is sancita karma (accumulated work). When a man leaves the physical body, he carries with him his astral body of seventeen tatvas and the karmasaya as well to the mental plane. This karmasaya is burnt by the highest knowledge obtained through asamprajnata Samadhi.
Modification is a vrtti in the mind. When the vrtti subsides it assumes the subtle state of potency or samskara in the citta or sub-conscious mind. This potency again becomes a vrtti under suitable conditions, when the mind comes in contact with sensual objects or through memory of objects from stimulus within. There are degrees in the vrttis or modifications that arise from the mind. You can check or control the lower vrtti with the help of higher vrttis or transformations. For instance, anger is a lower vrtti or a vrtti of a lower degree. Ksama (forgiveness) is a vrtti of a higher degree. With the help of ksama you can control the lower vrtti, anger. You can become a yogarudha (one who is established in yoga) by controlling all the vrttis of the mind. The yogi does not annihilate himself by controlling all the vrttis or modifications of the mind. By suppressing the transformations of the mind he acquires great powers. He becomes an (adept or perfect being) siddha.
Although the mind is one, it passes into many conditions or states, as it is made up of three qualities (gunas) – satva, rajas and tamas. All these qualities enter into a variety of combinations. The modifications or vrttis of the mind are also various. Peace of mind is a satvic vrtti. Lust is a rajasic vrtti. Laziness is a tamasic vrtti.