One reason to have a good body Jab in your arsenal is to slow down an aggressive opponent, who is pressuring you. It can also be used to get them to react to a level change by dropping there guard down to create an opening for the strong Cross to the head. If you are shorter than your opponent, this is a great weapon to setup an overhand right. They are used to defending the body, which for you may be eye level.
Protecting you while jabbing to the body, requires you to put the non-punching hand in front of your face and bending down at the waist slightly to ensure the body shot is straight and not exposing your chin. Slightly lean off to the side with your lead shoulder forward, similar slipping a jab. That is one reason why it is an effective counter to the Jab to the head.
The target of the Jab is anywhere below the chest. The popular targets are the solar plexus (below the center of the chest) and the liver, on the right side of the body.
1) The best setup for the Jab to the body is first Jabbing to the head, mainly to distract your opponent. (1.1B)
2) Feinting first to the head, so that you wont be easily countered. (F1.1B)
3) Countering the opponents Jab to your head with your own Jab to the their body.