The rear hand jab is a vital tool that few boxers develop. This is an important setup for a finish against a fighter stuck against the ropes. It is also clutch for a strong hooker who likes to stand up straight. If you are a tall hooker, then learn to lay in a straight rear hand jab to stand up a shorter man.
The rear hand jab is just an arm punch. You are staying mobile here and not fixing your hips with a weight shift, just popping him with a straight arm punch to settle him down and target. Imagine just pressing the palm of your rear hand against his chest to set him back against a wall. That is essentially what you are doing here, using a door man/bouncer compliance tactic. This works really good against a squared up puncher or peek-a-boo fighter whose legs are getting tired.
If you are a big stand-up fighter use a rear hand jab to the body to stop the smaller man from moving so much.
If you are a short peek-a-boo style fighter use some of your dips to power a popup rear hand jab to the chest to fix your man’s position for your hook.
How you use the rear hand jab is dependent upon your other tools. But before you get too far along in the game you should introduce it in the gym and see if it has any utility for you. Predominantly it is used by the stronger, less mobile fighter. In self-defense and MMA situations it could serve as a post to stop his forward momentum, upon which you can check or jab with the lead and get out of his wheelhouse or avoid the clinch.