Ok so I hope this helps. Photo 1 is the back of the stacked blocks. The lip has to hang over and fit snugly against the one under it. It causes them to be staggered in an over hanging way. Like an upside down stair case. The next 2 photos are of a block from the side. I'm pointing to the lip and you can see the size of it. It's not huge but enough to hold any horizontal pressure. But if there is some lifting and pushing then the block could be moved.
Let me know if my explanation isn't clear or you have more questions.