Yoga is the method through which we as a consciousness, as a perceiver, can unite our perception with facts, with truth, with reality, so we can cut through the illusions, the obstacles, in order to understand something real, something true. The way yoga approaches this is exactly the way we just discussed, through these three aspects: mind, heart, body.
The first teaching that Krishna gives in The Bhavagad-gita is how to act, how to use your skill, your energy, your body, to act; this is called karma yoga, which is simply how to behave, how to work, how to be engaged in the world, how to take care of responsibilities, how to perform any action — even mentally or emotionally, not just with the hands, the body.
The second teaching that Krishna gives is called bhakti yoga, and this explains how to use the emotional brain, related with the heart.
The third teaching that Krishna gives is called jnana yoga, which explains how to use the intellectual brain in your head, which is related with the intellect, the mind.
Unfortunately many people who study yoga take these teachings superficially and literally, and they think that they should study only one of these. We find this literal interpretation in every religion, not just in Hinduism.
The problem is that people only want to do what is comfortable to them: the intellectual types want to follow the intellectual approach to religion, while the devotional types want to approach religion devotionally. And the “action” types don’t want to study or be devotional, they want to act: doing service, or pilgrimages, or exercises, etc. In each case, the result is that they do not change or grow. They do not balance their three brains. Instead, they just keep behaving as they always have, with the same weakness they have always had. Later, they blame the religion for not giving them what was promised.
So, there are many intellectual people who only want read and study their scriptures. They really are only interested in jnana-yoga. They could be a Christian, a Jew, a Muslim; they do not want to change the way they live their daily lives, how they act and engage with others, they do not want to do anything emotionally, like prayer, they are not interested in these things, but they just like the ideas, concepts, philosophies, etc.
Likewise we find people who just want to go to church or temple to have that devotional, emotional connection, to sing songs and feel like they are getting something good from the church, temple, priest or lama; and they need emotional food. But they do not really want to study anything, and they do not really want to change how they behave.
Similarly, we find those who just want to do a lot of rituals, they do a lot of prostrations, circumambulations — whatever type of physical actions their religion promotes — but they avoid the emotional and intellectual aspects.