From Sensei Michael
Atemi Waza ("breaking")
The first three principles for successful breaking are:
(1) tighten the striking surface of the hand (or elbow/knee/foot)
Mental preparation is also ki. Hanshi recommends visualizing the target as something extremely fragile or soft (a stack of crackers or block of butter) - as opposed to something impenetrable.
If you convince yourself the target must break, it surely will.
Domo Arigato, Hanshi,
Michael Mackay, Yon-dan, SRKUSA