There have been multiple threads asking if the snowspeeder cable takedown on the AT-AT was a cutscene or if it was controllable, and I always posted this:
Once the Walker defenses are down you can also try a trickier but incredibly stylish move: the Tow Cable Takedown. Star Wars fans know the score: find a Snowspeeder and use its Tow Cable to entangle the AT-ATs legs, causing a fatal fall for the Walker. This is a real challenge, as damage to your vehicle can interrupt the takedown, and you also need to stay closer and closer to the AT-AT for each turn you take around it. Saving the day this way though will be a thrilling heroic deed.
http://starwars.ea.com/starwars/battlefront/news/defend-or-bring-down-at-ats-in-walker-assault-mode
I took this to assume you were fully in control of the speeder and had to manually wrap up the AT-ATs legs,
I was wrong:
http://www.youtube.com/watch?v=DzsAyz5CoAw&feature=youtu.be&t=397
Levelcap's recent video he posted says the snowspeeder is not controllable during the attack, and that there is a bar on the screen and that you have to keep a little snowspeeder icon in a certain place on it.