With the RGB channel option selected, if you click on the lower left point and then increase it's Output value the image will begin to fade. If you increase the Output channel to 255, the image will be completely white. So you can tint up all the image's values by increasing the lower left's Output level and leaving the Input level at 0. Or you can tone down all of the values by leaving the Output value at 0 and increasing the Input value. Not only can you adjust the Input and Output values for both points, you can also add, up to a maximum of 16, points along the graph's line.
Lets take a look at an example.
In the Original photo there is a red wash across the image. All the values are shifted towards red. I used the a C urves A djustment to correct the photo. Layer Curves option. Click here to get a zipped version of the original.