The subject of alterations by its nature is as infinite as Tango. To alter means to modify, to change or cause to change in character or composition, typically in a comparatively small but significant way.
Typical alterations start with understanding changes of directions that usually happen through reversing forward and back crosses through repetition. Alteration let you change the direction directly from forward to back and from back to forward not only influencing the movement of follower but the whole couple.
Alterations give you the ability to smoothly transition between patterns and create your own ones while smoothly floating around the floor. If you have problem improvising Alteration will help you understand how to start so you are free from eternal prison of Tango steps.