and I see the volume adjust option in the menu for HiJack, but I dont get how it works?

It is a dynamics compressor for the player. You shouldn't hear a sudden gigantic difference when you use it, but it should allow you to leave the volume control alone more. Quieter songs will seem to sound a bit louder, and the quiet parts of some songs will be louder compared to the loud parts. It should be rather subtle.

Details in the series of FAQ entries starting here.