Boards actually do hit back

Bruce Lee said "Boards don’t hit back" — which made for a great movie line, but it ain’t quite so: Newton’s laws assure us that boards hit us back with a force equal to how hard we hit them. Fine as far as it goes, and not much help either. The trick ultimately is to do it fast and let the derivatives do work on the relatively inflexible materials we break while doing little or no damage to the highly flexible body structures we use to break with.
This esoteric description is neither necessary nor ultimately particularly helpful in actually breaking stuff — that’s purely about doing it, no matter what your brain tells you ;-).
Here are some samples of my breaking from my 3rd dan promotion [click’em to take a closer look].

This is one of the easiest hand techniques to get right — which this isn’t a very good example of 🙁 — because I just walked up and smacked it with little or no finesse. I can break three if I do it right, so sloppy was good enough for just the one.
Most newer students are awed/scared by breaking with knuckles, but it really is not all that bad so long as you go fast and don’t flinch at the point of contact. Again, only a single board, so no big deal. Two is a bit harder and requires good alignment to keep from skinning the hands and raising bruises. Three requires (of me at least) very good holders and some serious psych to get ‘er done.
This is an easy way to get a big pile of wood chips out of multiple boards so long as you nail the aim. Speed is easier to get out of the legs than it is the hands because the muscles are sooo much bigger and the distances you can reach significantly longer — V2 = 2*a*s really does work!
I do this one a bit different than most people because both big toes are so screwed up (from jambing them in tkd) that I have trouble getting them into the right position for breaking with the ball of the foot like is taught. I use the top of the foot, the instep, and have no problems breaking two boards this way. But speed is absolutely(!) critical.
The hilight of testing at all black belt ranks is concrete breaking. I have struggled with this off-and-on when required to do knifehand strike, but I have never failed to break with palm strike. The two breaks require radically different technique. Shown here are the three 8" by 16" concrete patio pavers still tumbling to the floor as I withdraw from the strike.