this should give u an idea of the falling step
taken from another forum

"The concept behind the falling step is this, think of stepping on a set of scales and jumping up and down on them, you'll notice that you're weight reading rises from say, 11 stone to 16 stone. In essence in that moment that you land on the scales, you transform into a heavyweight for a split second. And so the theory is that you punch with the force of an extra few stone.

To practice just imagine that when stepping forward when you punch(stepping into your punches is something you should always try and do with every punch) you are stepping and falling down a step.

Dempsey said that every straight punch shopuld be performed with a falling step"

its explained in Dempsey's book-its actually a very rare book and sells for at least £100

but some chapters are on this website

http://www.lousy.org/budo/local/dempsey/dempsey.html