Complicated storing of positions and calculating averages and splines. I have never tried it before but I guess you store all the values returned for a peroid of time, then try to calculate the shape that is formed.
basically you force the player to point at the screen and you use that as a reference point to determine where the player moves afterwards.
Saw this the other day;
http://procrastineering.blogspot.com/20 ... le-as.html