http://mootools.net/blog/2007/10/23/whats-new-in-12-the-best-fx-now-even-better/

Here are some examples of the link option in action.js

var myTween = new Fx.Tween('myElement', 'background-color', { 'link': 'chain' });
//fade myElement to red, then to white, then to blue
myTween.start('#c03').start('#fff').start('#369');

var myMorph = new Fx.Morph('myElement', { 'link': 'cancel' });
//transition myElement to red 16px sized text
myMorph.start({ 'color': '#c03', 'font-size': 16 });
//a few ms later... stop right there! make that blue 10px text with a height of 100px
myMorph.start({ 'color': '#369', 'font-size': 10,  'height': 100  });