The gp:FindMovedPoint function contains some very powerful LISP expressions dealing with list manipulation. Essentially, what this function does is compare the list of the current polyline points (after the user dragged one to a new location) to the previous points, and return the keyed list (the 13 <xvalue> <yvalue>) for the moved point.
The best way to figure out how this function works is to step through the code and watch the values that it manipulates. Set a breakpoint right at the first expression (setq result . . .) and watch the following variables while you step through the function: