Urgent help needed please. Tracing the outline with trailing effect

Look at http://www.voxunity.com/adcons/en/. There is this white thing that goes along the dotted lines, lady etc with a smooth trailing effect. Does anyone know how to do this?

Please note that this is not the effect i want-> http://www.newtutorials.com/cool-masking-effect.htm because this method only allows the white thing to move in one direction.

What i need is exactly like http://www.voxunity.com/adcons/en/ . The white thing has to be able to move along any irregular shape outline smoothly with the trailing effect(example moving around the outline of the shape of head).

I can’t seem to find a tutorial on it. Someone please help.