Teen Wolf Question

[switch842]

Finished season 1 and I have a question - what's the deal with turning? Obviously, the full moon does play a part in it, but it seems like they can also turn whenever they want if they get mad enough or whatever. Like, in the final fight with the Alpha, Derek and Scott both turned. Then in the scene after that when Scott and Allison were on her roof, the moon wasn't full. So, what's up with that? I don't recall it ever really being explained on the show.