Albert Barnes Commentary 2 Samuel 2:23

"Howbeit he refused to turn aside: wherefore Abner with the hinder end of the spear smote him in the body, so that the spear came out behind him; and he fell down there, and died in the same place: and it came to pass, that as many as came to the place where Asahel fell down and died stood still." — 2 Samuel 2:23 (ASV)

With the hinder end... that is, the wooden end, which was more or less pointed to enable the owner to stick it in the ground (1 Samuel 26:7).

The fifth rib... The word so rendered here (and in marginal references) means the abdomen and is not etymologically connected with the Hebrew for "five," as the translation “fifth rib” supposes, but with a verb meaning to be fat or strong.