Heel of Foot
The bottom of the heel is best striking surface for a kicking break. There will be a straight line of bone from the heel to the hip.
Ball of Foot
The ball of the foot is best striking surface when using a round/roundhouse kick. Using the instep will probably lead to an injury.
Head breaks are relatively simple but they are deceiving. Although there is no apparent injury, the neck vertebrae take most of the shock. Later in life, the discs between the vertebrae will cause problems because of blows received to the head in earlier years. I speak from personal experience.