Here is what happens if you play with the Step number.  In my own words, here is how I see it.  The lower the number, the denser the brush, the more solid the paint that gets laid down.  I think the Help book says it gets softer.  I guess you can think of it that way since the softer you make an image, the more it blurs together and sort of turns to mud.  (in my opinion.)  So look at the difference in the 4 images below.  Number one at Step 25, number two at Step 1, number three at Step 50, and number four at Step 75.