Well damn... maybe I hit it odd or had a crappy holder. Emboldened by Oldman's findings, I will try against the grain with an elbow, and maybe a palm strike or hammer fist. I'll also work on mastering my digicam so I can take a short movie of it.
What results did you get?
I had 2 people holding my boards.
I broke the boards twice along the grain instead of once across the grain with an elbow strike. I had one holder, which has never really been a problem. Hit them really hard, so I just kinda figured that (as Doctor X put it) they would fail along the grain before they succeeded across it.