Curves, long the backbone of image adjustment, continue to grow both more powerful and easier to use. In CS4, the new On-Image control in the Adjustments panel makes adjustments as intuitive as clicking on the image and dragging up or down; of course, a few keyboard shortcuts are worth noting when using the On-Image adjustment control:

• Preview circle on curve: Hover cursor over image (CS3: click-and-hold)
• Add point to the active curve: Click the image (CS3: Command-click [PC: Ctrl-click])
• Delete point on active curve: Command-click (PC: Ctrl-click) image in area where Input/Output values of point are the same (preview circle will appear over point in curve)
• Select the next point on the curve: Press the + (plus) key (CS3: Control-Tab)
• Select the previous point on the curve: Press the - (minus) key (CS3: Shift-Control-Tab)
• Toggle through channels: Option-2 through 5 (PC: Alt-2 through 5) (CS3: Command-~ through 3 [PC: Ctrl-~ through 3])
• Preview clipping: With the Black or White Point Eyedropper tool, hold Option (PC: Alt) and mouse over the image