A unit of Savage Orcs that includes Big Stabbas and has two or more ranks of at least five models has the Impact Hits (D3) special rule. Nominate a single model in the front rank of the unit to be the one that inflicts the Impact Hits. The Impact Hits have a Strength of 5 (no matter what the Strength of the model that inflicted them), and against any Large Target also have the Multiple Wounds (D3) special rule.
Note: Per the Orcs & Goblins FAQ, the Choppas special rule applies to the Impact Hits caused by Big Stabbas.