Brush 'Direction' option, greyed out

Under the General tab for brush control, the 'Direction' option is greyed out? If I tick the expression box amd apply my Wacom pen, when I increase the pen pressure on the tablet the paint fades out. I want to reverse this action. What governs the fact that this option is greyed out? Anyone know.

yogi62

update: sorry guy's, I should have checked the 'Help' menu properly first. The 'direction' control is not what I thought it was for. It would appear that simply leaving the box unchecked does what I want.