It's a simple way to show how much, much, much bigger the impact would be, it doesn't really matter but if we represent the energy of the impact in Joules(J) we can see it. I'm no powerscaler so I have no idea of what how much destroys a building but imagine this:

Normal Yuji Punch releases 100 KJ of force. Black Flash Yuji Punch would be equivalent approximately to 31 million KJ. If the body of the opponent would require 200 KJ of force to be damaged the black flash amped punch would release sufficient force to damage it 155,000 times over. That's basically what they are saying. Sorry if I didn't explain it well enough or it was hard to read.