The heart is the center, the core, where everything begins and ends. In martial arts, the heart sits at the the intersection of form, intent, action, and reaction. In Cabal Fang martial arts, these four things equate to the Hermetic Quaternary -- "To Know, to Will, to Dare, to Keep Silent."
These associations are prehistoric and are equated to the four directions. Thus we see them cropping up everywhere -- the Four Living Creatures from Ezekiel, the four heavenly creatures (Ox, Lion, Man and Eagle), the Four Holy Beasts from Vietnamese folklore, the Four Symbols from Chinese folklore, etc.
Heart: Martial Arts Training Involution #178
Form. Spend 15 minutes working on your form. How do you do that? Here are some ideas: break down a technique by practicing it slowly, get in front of a mirror and analyze your movement, practice your kata, poomse, hyung, etc. with extreme exactness, etc.
Action and Reaction. Spend 15 minutes working on your action and reaction. If you have a partner, work flow drills. If you're going solo, run flow drills with your heavy bag, grappling dummy, floor bag, etc. (if you need some flow drills, read Chapter 26 of the CFSG).
Intent. Spend 15 minutes honing your intent with meditation. Intent is the secret sauce that makes everything you do open up like a flower (last week I explored a related idea in this video). Think of a suitable phrase, about a dozen words, that embodies your intent. Pick a famous quote from your choice of wisdom literature -- a Bible passage like the one I used ("Be ye therefore perfect as your father, which is in heaven, is perfect" ~Matthew 5:48) or a quote from the Tao Te Ching (“Those who know do not speak. Those who speak do not know"). Set a timer for 15 minutes and assume your meditative posture of choice. Mentally recite the first half of the phrase as you breathe in. Hesitate with lungs full and airway open as you mentally recite the second half of the phrase. Recite the first half as you breathe out. Hesitate with lungs empty and airway open as you mentally recite the second half of the phrase. This phrase, when split and used in this manner, will focus your mind and urge you into box breathing. Repeat until the timer beeps.
Record. And, as always, record your results and thoughts in your training journal.