Let's see what the user defined filter does: Go to Effects || User Defined. There are several sections for this filter: the Filter Matrix, Matrix Modifiers and the Apply To.
In the Filter Matrix section there are 49 boxes, all set to 0 but the middle one that is set to 1, if you click on the Default button in the top right of the dialog. In the Matrix Modifiers section you have Divisor and Bias and a Compute button. The Compute button is your friend. Then in the Apply To section you have Color Channels, Grey Values, and Red, Green, and Blue.
Randomly pick boxes and put in numbers, from -999 to 999. As many or as few as you like. Your picture will most likely disappear, be blurry or simply fade out. Don't panic. Now click on the Compute button. Depending on where and what values you have put in your picture will change.
Now let's look at the Bias factor. As a former seamstress (before PSP) I thought of bias as stretching the fabric. The bias works in much the same way. Pick a random number for this box (from -100,000 to 100,000). Negative numbers make it darker, positive numbers make it lighter.
In the Apply To section of the window, click on Grey Values and you will see that Red, Green and Blue are no longer available. Click back on the Color Channels, then uncheck Red. You'll see that the red values of your image have all been greatly altered.
Put a check mark back by Red and uncheck one or two of the others. You will then see how this affects your picture. Now let's reset it to default and see what we can do with this picture we have.