The slick makes the wheeling machine into more of a slip roll. Two hard wheels will add some stretch, where the panel will show crown in both directions. (lengthwise, sideways) Using the slick, and the lower die pushing up into it (as it's only partially inflated) the panel as it goes through assumes the radius of the lower anvil (sideways direction) and remains all but flat lengthwise. Think nice long rocker panel.....