Vaan_Hatake
Exactly as Quiet_Way said, all technique power in Shotokan derives from the subtle or not so subtle (Depends on technique used) rotation or vibration of the hips.
And I have to add, you don't want to be on the wrong end of it. To continue the prior comparison, boxers tend to aim for the head, or the midriff. A shotokan practitioner, however, appears to aim about five feet or so (exaggeration, I know) behind you. Thus, by the time the strike lands, you are not only punched, but typically driven backward. And if you don't give (say your legs are exceptionally strong, perhaps, or you have a good root) then... well, that's why they demonstrate breaking boards. I have five persistent knuckle bruises running across my sternum that remind me to get out of the way when that punch comes.