media compensation, basically put, is the amount of media that is sent through the printer each pass. The compensation is measured in pixels ( I believe) so unless you are inputting significant numbers, and changes you make are difficult to see with the naked eye.
Depending on your setup mechanical media compensation "CAN" be over ridden by your RIP initially, afterwards you can change it on the fly on the keypad.
incorrect media compensation exhibits itself, but either dark, or light bands in your printed material. If you have dark bands in your prints, you need more compensation, light bands, less.
To change on the fly, hit function, you will see media comp come up, the larger then number the more media passes through, the smaller, the less. Change the number accordingly, hit enter and wait until the head makes a complete series of passes to see the difference ( ie: if you are printing in 8 pass mode, let it build 8 passes to see the difference)
Hope this helps.