The prescan station is used to adjust the scanner settings while controlling the scan. Start scanning and you'll see the scanned image be displayed on the fly.
With the scan direction buttons 
you can scan forward, backward or forth and back on a certain area. You point out this area by dragging a rectangle with the left mouse button on the image in the image window. Having selected an area of the drawing you can experiment with different scanner settings to get the best scanner result for your drawing. To zoom out click with the right mouse button on the image in the image window.
If you want to restart or eject the scan you can use the buttons: 
When you are satisfied with your scanner settings, click on either the scan- or copy station icon to proceed.