Simplify the following text for a Grade 9 native English-speaking person.
Original: An example of the union of mental and physical discipline is the breaking of wooden boards , which requires both physical mastery of the technique and the concentration to focus one 's power .
Simplified: Breaking boards , for example , demonstrates a union of mental and physical discipline . Breaking a board with one 's hand or foot requires both the physical mastery of the technique and a concentrated focus on one 's force .