training unarmed

[Thawwing Light]

Pretty much all the hunter melee skills will.  I usually start my melee auto-attack by using raptor strike.  Charge and several other trigger it for warriors as well.  Actually anyone with a special melee attack can turn on auto-attack with it.
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I can confirm Sunder Armor triggers auto attack - I tend to use that instead of any attack key, unless I charge.

I'm pretty sure Rend triggers it as well.