This is probably a complete noob question, but due to my general lack of experience in this area I wasn't able to find the answer on my own. When you want to reroute a signal on the PCB to an external location, or replace a signal with one of your own, what situations do you need to cut the signal off from an existing path? For instance, in this example, a BIOS chip is successfully overridden by soldering a new chip over the existing chip with no cuts made. In many console modchips (although I don't know what their precise functionality is) many signals are rerouted and modified just by soldering to the surface of the PCB. But in this example says you need to cut the pin to replace the clock signal. So why exactly is it that in some cases the signal can be overridden by soldering over a connection, and why is it in others you have to cut the existing connection before you can add a new one? Also, is it possible to pull off the mod in the last example without making any irreversible cuts?