For the full series see http://tale2tell.com/after-effects-basics-tutorials-to-get-you-started-with-adobe-after-effects/

Love this tool – easy to use, with a few powerful options this tool allows you to ‘write on’ and ‘write off’ any shape you have created be that simple shapes, text outlines which have been turned into shapes or even open-ended shapes created with the pen tool. Great fun – but you really need to use this tool with shapes that have the ‘Fill’ turned off otherwise they can look really odd when animating.