I would very much like to see a video of how you do these breaks.
I've love to provide one, but none such exist at the moment. There was a nice one that one of our student's moms took of me breaking a single paver at a demo a couple years back, but her child no longer trains with us and she didn't share the video she made.
I will try to describe as best I can.
Pavers are arranged horizontally across two upended cinder blocks of the common size. The cinder blocks look like yours in your avatar, but on end, so a bit higher. We generally place a duct-taped phone book over the top cinder block. We do not use spacers between the pavers, just one on top of the other.
Most, but not all, students begin from the standing position. They address the pavers standing bolt upright, not in a fighting or deep stance. They lift the hand they intend to use to strike with overhead and at the same time, they drop to one knee. As they drop, the hand drops as well. The moment of impact occurs as they reach the 'one knee' position, so the arm is stretched out nearly horizontally at impact. The strike is with an open palm. It could be described as looking like a slap, but it is not an actual slap; this would disperse the energy. The energy of the strike should be traveling downwards through the pavers. We are told to think about striking well through the pavers, rather than striking the surface.
When unsuccessful, there is a loud 'slapping' sound and the palm stings like crazy. When successful, the sound is a deeper 'thud' and the hand passes through the pavers as they collapse. I generally end up with the broken pavers on top of my hand and I have to use some caution extricating it.
I have a bad knee which gives me trouble at times, so I begin from a kneeling position. This forces me to use technique over power, since I cannot generate a lot of power from a kneeling position. I raise my right hand overhead and bring it down sharply, but not with monster speed or power; I am concentrating on technique, on passing my arm through the pavers.
In either case, we are told to think of our striking hand as if it were an elephant's trunk. Supple, flexible, coming down in sort of an S shaped movement. It has been described as a hand holding a iron ball, or like a waterfall, but it's all mental visualization for the most part. The physical part involves the shape and movement of the arm and the 'thud' of the impact, as the power is transmitted through the pavers.
We have noted many times that sometimes when a person is unable to break multiple pavers, despite a 'good' hit, the bottom paver is the one that breaks. Occasionally it will be one or even two in the middle of the stack that breaks first. Never the top paver. Often when we attempt the break and only crack or break the bottom paver, a second strike finishes the stack.
It is a bit hard to not visualize what we do as a hard or power break, but rest assured we are not doing the traditional downward-facing power strike (as in your avatar). But neither are we doing an 'iron palm' break as seen in many youtube videos.
For example, we set up the pavers as seen in this video:
But we don't break like that at all. The break seen at 39 seconds is a power break from my point of view.
We also do not break as seen in this video:
The above is the 'slapping' technique you described, where they do not pass through the pavers. We are seated like that, our palm is in that position, but we raise the arm higher, drop it slower, and we move through the pavers, we don't 'bounce off' of them.
Much closer would be this fellow. His arm moves just like ours does. He bounces off the top, we don't, and we have the pavers set up lower, like the videos above, but the striking motion he uses (slow move, elephant-trunk shaped arm, and palm strike) are pretty much identical. We just move through the pavers instead of bouncing off.
I hope that this helps instead of being confusing. The one video that did exist of me breaking a single paver looked like the fellow above, except I was on my knees and my hand passed through the paver.
EDIT: I should also say that breaking pavers is not an integral part of our training. It's something we do for fun maybe twice a year or so. Builds confidence, more than anything else.