(...) it is carried instead of a normal hand weapon. (...)
An Orc warrior on foot armed with a single choppa adds +1 to his Strength in the first round of combat when he charges. This bonus does not apply if a weapon is carried in each hand, or for Boar, Chariot or monster riders - in such cases the weapon's natural brutishness is cancelled out by the difficulty of wielding it.
The Choppa bonus applies to character models on foot that are armed with a single ordinary non-magical choppa. It does not apply to individuals armed with magic weapons as they receive the appropriate magical bonuses instead.
(...) only Orcs on foot benefit from choppas (...)
(...) Note that the bonus armour save for using a hand weapon and shield does not apply to an Orc wielding a choppa. Foot models with a choppa in each hand (additional hand weapon) do get normal +1 Attack for having two "hand weapons", though the lose the +1 Strength on the charge. [p. 6 of AB]