Hi Boyla,
First off, the liver is an organ that's on the right side of the body, primarily beneath the lower right side ribs. It's a target that's definitely been ring-proven to be a potent one to attack. The best punch to attack it with is a left hook, specifically a lead left shovel hook (i.e. from an orthodox stance), as part of a combination. The best punches to open up the liver for a clean shot are: 1) a left hook to the head, 2) a straight right hand to the head, 3) an overhand right to the head, 4) a right uppercut to the head or body, 5) a left jab to the head, and though it's not traditionally/officially recognized as a punch in boxing parlance, 6) a rear right hook to the head or body.

As far as using an uppercut (rear or lead) to the liver, ahhh, that's not a punch to use in this case because its angle/trajectory of application doesn't mesh with where the target is situated. Save this punch for the solar plexus and chin.

When I was an active amateur boxer long ago, I was primarily a headhunter (as influenced by Ali and Pryor, much to the chagrin of my coach). I mainly threw in body shots during barrages when my opponent was wholly preoccupied with covering his head from the onslaught. My objective for doing this was to simply distract his focus and draw his arms down so as to open his head back up for me to attack cleanly. I also occasionally incorporated them in two to four-punch combos when adamantly directed by my coach to do so.

Anyway, if you want to see some masters in action, go to YouTube and watch Micky Ward, Julio Cesar Chavez, and Thomas Hearns in action. These three had vicious left hooks to the liver (among other things). Heck, while you're there, check out footage of Roberto Duran, Mike McCallum, and Alexis Arguello too. They weren't too shabby either...

Take Care,
Lito