One of the ways to see how our feelings affect us is to observe our heart rhythms. Feelings that are unpleasant and intense such as anger, frustration, angry etc., show up as uneven and jagged heart rhythms. In contrast, when we feel positive, appreciative, confident, secure etc., our heart rhythms are smooth & even.
These heart rhythms relate to how are sympathetic and parasympathetic nervous systems are working together. Generally these two branches of our CNS are out of synch and are working like having one foot on the gas pedal and one foot on the brake. When they are in synch, they manifest by being smooth and even. This is accomplished by combining our physical effort of belly breathing with our positive thinking.
Synchronicity achieved by combing belly breathing with positive thinking is superior to just relaxing (which is helpful but could be so much more). With HeartMath, you learn to take control of your emotions and as you become skilled at it, you increase your level of well-being, become more self confident and satisfied in yourself and in your relationships.
The heart is involved in how we feel & how well the brain integrates information to various parts of itself.

Research shows the following consequences of unhealthy emotions:

Less ability to think clearly
Less efficiency in decision making
Less ability to communicate clearly
Reduced physical coordination
Higher risk of heart disease
Higher risk of high blood pressure

With practise, you can learn how to generate positive emotions for yourself.
Research shows the following consequences of healthy or positive emotions:

Improved performance and achievement
More creativity and innovative problem solving
Better decision making
More flexibility in the way you think
Improved memory
Improved immunity to disease
Improved hormonal balance
Longer life span (HeartMath)

When we tend to belly breathe through the heart center and generate a positive feeling, we can change the signals the heart sends to the brain, influencing the Brian’s perception and improves how we feel.
Information in the brain is also regulated by the thalamus. One of its roles is to distribute incoming sensory information to different parts of the brain and to see that they are synchronized. When the information coming into the thalamus is in keeping with a jagged signal, it interferes with the thalamus’ ability to do this job. Positive feelings, however, give way to smooth signals to the brain improving the brain’s ability to process information. This helps with cognitive processing of information and making good decisions as well as improving the brain’s functioning for coordination and speed.