You can try mixing a riff down with nothing but the instant drummer on it using the mix button. Then pull it back into your song using import. When you see the riff appear it will be called bounce. If you press the info button on the riff you'll see a pictorial view of the wave file. You can click and drag the line near the top of the wave file and fade in or out, or mute sections completely. So you should get a version of what you're after that way. Hope this helps.
Uncle Bill
