שמואל ב, פרק כ׳, פסוק י׳

A sudden and fatal assassination is carried out with cruel precision, exploiting a victim's complete lack of suspicion. Immediately afterward, the killers resume their military mission as if nothing has happened.

Amasa fails to protect himself because he simply does not perceive an immediate threat. Having seen Joab's sword fall from its sheath naturally, and watching Joab casually pick it up with his left hand, Amasa remains entirely off guard [מצודת דוד, ביאור שטיינזלץ, רד״ק].

The fatal blow is aimed at a critical area. The primary approach among commentators is that the strike hits around the fifth rib, where the liver and gallbladder are located [מצודת דוד, רלב״ג], or the lower abdomen [ביאור שטיינזלץ]. Another perspective suggests the target is actually the most naturally fortified part of the body—the heart, which serves as the seat of life and the source of the body's natural heat [רלב״ג].

The strike is delivered with such immense force and depth that Amasa's intestines spill out onto the ground [מצודת דוד, ביאור שטיינזלץ]. The attack is so devastatingly effective that a second blow is entirely unnecessary; the initial strike is instantly fatal [מצודת דוד, ביאור שטיינזלץ].

Without pausing, Joab and Abishai immediately return to their original orders to crush the ongoing rebellion [ביאור שטיינזלץ]. Both brothers fully commit to the pursuit of Sheba the son of Bichri, with each man independently driving the chase forward [מצודת דוד].