It's an interesting way to practice delivering power. I'll have to see if I've got some more bricks in the garage and try the knee drop.
Of course, I don't even know if I'm actually doing it right, since it's not a technique taught in our school.
Have you tried it with boards?
No, we don't break boards in our dojo.
If I recall the instructions we are given correctly, it is this:
1) Loosen any tight clothing and/or remove obi.
2) Visualize reaching past the concrete to the floor below (sometimes Sensei will put a dollar under the bricks and tell us too imagine we're just reaching for it).
3) Raise hand overhead and let it fall like an elephant's trunk or a waterfall (I have lots of trouble doing this).
4) As the arm comes down, drop to one knee, leaning forward. The blow gets delivered with the arm out and the head tucked down.
5) Visualize the hand holding a giant iron ball bearing or something like that, which you visualize crashing through the concrete.
Sensei notes that if the sound is a deep dull thud, that's correct. If it makes a slapping noise, that's not correct. It is not unusual for the bottom brick to break, leaving the top bricks. It may or may not fall down, but you can see it fractured.
I've seen big bruisers unable to break two bricks and petite ladies who broke 3 and 4. When it is done right, it looks effortless and generally catches the person who did it by surprise. One thing I always notice is that their hand never rebounds from the bricks; when they break, the arm goes straight through them and you end up with some cement on your arms. It's pretty clear that they are aiming through the bricks, even if they don't realize it.
I'm no expert! And like I said, we only do it a couple times a year, when Sensei is in the mood.