Certain stocky races excel at using their bulk to more easily handle large, heavy weapons. The powerswing racial increases your chances of using 2nd attack (and similar skills) while wielding 2-handed weapons.
To determine whether or not a weapon is affected by Racial Powerswing, simply Examine it while trained in Weapon Lore. To gain the benefits of Racial Powerswing, you want to see the following in the Examine output:
It is a two handed weapon which does (type) damage.