By default, changing the seed for the Julia set requires recalculation of the image (which is quite slow). It's a nice effect to change the seed smoothly and show the Julia set morphing as the seed changes. XaoS has a special algorithm which can calculate such morphings in realtime. It is very inexact, but it is good enough for a fast preview.
If you want to select a good seedpoint, enable fast Julia mode and find a nice place by dragging with the first mouse button depressed; then change to the Julia mode to see the exact image.
Available as: menu item, command line option, command