It has been proven that Heart and Brain beat in unison: If our mind and emotions affect the heart rate, heart rate also has an influence on our brain.
By controlling you heart rate, you can also control your emotions, limiting your overall stress condition.
The easiest way to control your heart beat is by controlling your respiration.
Mindfulness is about paying attention to the present moment, thoughts, emotions, physical sensations and the environment in a deliberate and non-judgmental manner (eg, "This emotion is incorrect"; This feeling is undesirable ").
A slower, controlled respiration directly reduces and control the heart beat.