Addicting, well worth the $5, highly recommended.
This is done by drawing a flight path from each aircraft to its landing point. The plane will then follow that path. You can re-draw the path for a given aircraft at any time.
The fun/challenge is in avoiding conflicts between aircraft of different speeds, and having to think ahead in terms of where things will be, not where they are now. You may have a plane on approach to its runway and a new faster plane shows up and you may be able to sneak it in ahead of the previously #1 aircraft.
There are generally three classes of aircraft (large, small, and helicopter) each of which has its own landing point. The large aircraft come in multiple sizes/speeds.
It's a simple game mechanic, but I find it fun, satisfying and addictive.
The stylus is exactly the right control for this, much better than the finger-operated iPhone version.
A great downloadable for your DSi.
G.