Breaking the bottom block in a stack falls under what is known as ‘selective breaking’. One must focus the strike to break only certain blocks in a stack. It is a tricky technique and, frankly, not one I have mastered. I don’t know if it is directly applicable to fighting, but it is yet another way to hone your control.
For me, a true demonstration of selective breaking must be performed on a top or middle block. Simply breaking the bottom block is not enough. In my experience, the bottom block has a natural tendency to break first. It is on the bottom after all, so it absorbs the brunt of the attack, especially if the blocks are not perfectly formed.
Irregularities inherent in blocks can prevent the top block from resting perfectly on its edges, causing more pressure to be focused on the center of the bottom support. It is not uncommon for the bottom block to break during a failed attempt at two or more blocks. What’s more, variations in the strengths of the blocks can result in random breaks in a failed attempt. For instance, one may not break a stack of four, but upon removing the blocks, discover that the first and third have actually been cracked. Some would describe this as an unintentional selective focusing of the chi. I think that is a misguided leap in logic. Whether or not one believes that chi may actually be focused in such a way, attributing all manner of oddities to it, detracts from true application.
This particular kind of break is a simple matter of physics and variations in block strength. On a related note, blocks can be weakened with strikes without breaking. You may take a block you have weakened with previous strikes, give it barely a touch, and it falls apart. An exhibition of chi? No, just a weak block.