Charles Ellicott Commentary 2 Samuel 20:10

"But Amasa took no heed to the sword that was in Joab`s hand: so he smote him therewith in the body, and shed out his bowels to the ground, and struck him not again; and he died. And Joab and Abishai his brother pursued after Sheba the son of Bichri." — 2 Samuel 20:10 (ASV)

In the fifth rib = Abdomen. (See Note on 2 Samuel 2:23.)

So Joab and Abishai. —Joab here comes forward as the commander of the pursuit without previous mention. He may have accompanied Abishai from Jerusalem, or he may have joined him on the route; but, now, having murdered Amasa, he assumes his old place as commander-in-chief, doubtless with the connivance of his brother.