There's no question that the thought of being forced to sever a limb from a friend or family member—or even yourself—is a truly horrifying thought. However, in a worst-case scenario, an emergency amputation may be the only way to save a life. So, it's good to know how it's done, even if there's a 0.0001% chance you'll have to use that knowledge someday.
Before you read on, know that we are certainly not recommending this procedure as anything but an absolute last-ditch effort to save a life. Otherwise, don't take unnecessary risks, and leave medical procedures to professionals. That said, here are four situations when emergency limb amputation might be necessary, based on an article from the Sydney Helicopter EMS web site:
Here's an infographic that shows a technique for emergency amputation, based on a simple method used as far back as the Civil War. The art style is somewhat comical (note the jar of leeches), but the technique is surprisingly legitimate.
This method leaves flaps of skin which can be sewn together using a glover's suture to close the wound. Cutting through the bone can also be done with a wire saw, as seen in this video where the leg of a deer carcass is amputated.
Obviously, do everything you can to sterilize and disinfect the wound and your tools to limit the risk of infection. Tying off large blood vessels or applying a second tourniquet may be necessary to stop blood flow. Also, we'd strongly recommend applying the best anaesthetic you have available (in this case, brandy) before surgery—because it's going to be very painful.