I thought we had a tutorial for this and then I remembered I did this in person in a hotel room full of JC guys the night before Joe Con while hotgluing a Ninja Force figure to foam board for a deconstructed view. It works 80% of the time.
Step 1:
Cut the o-ring. Don't worry, you'll replace later if this isn't just for fodder.
Step 1a:
At this point some people swear by freezing the figure until the plastic is cold and brittle. Others swear by sticking the figure in boiling hot water to make the figure softer and less likely to crack. I personally haven't had more or less success one way or the other so I don't do either, but just so you are aware those are additional options that may work for you before continuing.
Step 2:
Place the upper body into a small ziplock bag. Trust me on this one, it will keep the parts from flying all over the place in the next step. No one wants to be on their hands and knees looking under furniture for that torso back somewhere.
Step 3:
Using needle nose pliers, insert into the hole where the o-ring comes out of the torso. Slowly and gently, pry the pliers open until you feel the plastic pry apart and eventually pop. Because the torsos are glued together, you may have pry part of the way and readjust the pliers deeper into the torso. Eventually, they will pop apart...sometimes in a symmetrical front and back torso way like you want...and sometimes cracking the front or back in a jagged edged line in the process like you don't want.
Warning: Some Ninja Force, Mortal Kombat, and Street Fighter torsos have internal gears and springs in them that the bag will also help keep from flying all over the place. These are small and do tend to go in every direction so be prepared.