You don't "make a lag switch." You DL the program and run it through your network to flood it with UDP packets. The host server tries desperately to synchronize both clients, which is why the game sometimes appears like a slideshow. It really is that easy. We have to run UDP flooders (lag switches) at work semi-regularly to simulate DDoS attacks.
It depends on the game though. Some games a physical switch does work very well just by adding in an artificial delay. But yeah, most people use software I won't mention here to accomplish this.