If you were to clamp a barrel into a bench vice with metal jaws, then tried to solder right in the middle of where it is held tightly, the vice becomes a heat sink, drawing the heat away
& you will have a devil of a time getting the barrel hot enough to solder anything to it. That would be a heat sink where you don't want it. By the same token, you could solder on either side of the vice, without effecting the joint you just made on the other side. The vice would act like a heat sink and protect the first soldering joint.