The Clip Preview Window lets you check out your clips properly

You can select which clip is previewed by clicking on clip thumbnails in the layer stack or the main thumbnails. The main scratch slider below the clip preview window lets you see where you're clip has got to and lets you scratch. Press P to pause a clip to let you scratch it. Once you're scratching a clip you can unpause it to get AV jump scratching.

The scratch slider has 2 smaller sliders above and below the main one. These are the IN and OUT point markers that you can use to choose new AV loops from your clips on the fly.