I may look for it, but in principle on the resistive screens the point is the average between two points of contact:

you put one finger in X, you save the position
you put second finger in Y, for the device the point of contact will move rapidly to (Y-X)/2
by moving your second finger you change the second position relatively to the first one

I hope you understand what I mean If not, I will try to search for any video demo, as I know it exists. You can always try this on your resistive touch device (if you have any) in you drawing program.