The sword that was in Joab's hand; which falling out, as it seemed, casually, he supposed that Joab intended only to put it into its scabbard, and therefore took no care to defend himself against the stroke. In the fifth rib: See Poole on "2 Samuel 2:23". Struck him not again; he despatched him at one stroke, or gave him his death's wound at the first blow, and needed not to smite him again. Joab now boldly resumed his former place, and marched in the head of the army. It is not strange that Amasa's soldiers did not fight to revenge his death; partly because not many of them were yet come up, but came by degrees, as the following verses show; and partly because Joab's interest and authority with the military men was very great, especially with David's guards, who were here present, and who had no kindness for Amasa, as having been the general of the rebellious army; and, as they might think, was not fit to be put into a place of so great power and trust.
Related Commentaries of 2 Samuel 20:10
2 Samuel 20:10
10 But Amasa took no heed to the sword that was in Joab's hand: so he smote him therewith in the fifth rib, and shed out his bowels to the ground, and struckd him not again; and he died. So Joab and Abishai his brother pursued after Sheba the son of Bichri.