Hey, this is something you want to do research on for the future.
The use of tourniquets, tight straps around an arm or leg and tightened with a windlass to stop bleeding, have been around for nearly 400 years. The first documented case of a tourniquet used on the battlefield was in 1674.1 Discussions of constricting bands (but without the use of a windlass) date back much further than that.
Tourniquets are absolutely essential on the battlefield. They provide a hands-free way to stop bleeding, which gives the soldier the freedom to keep fighting and prevents death from hemorrhage. From 2001-2010, tourniquet use by combatants increased, along with survivability. At the same time, injuries got worse. Tourniquet use became the gold standard and every US Army soldier was taught to use them. Every US military person has issued a tourniquet when they entered a combat area.1
This being emergency medical services, we never let a lack of evidence dissuade us from our beliefs. After military evidence from combat in Iraq and Afghanistan began to pile up saying tourniquets were safe and effective, civilian paramedics sat up and took notice.
Tissue damage—usually localized to the area where the tourniquet is applied and not the entire limb—does happen. But it's not a one-for-one swap, life for limb. There is very little evidence that emergency use of a tourniquet causes any significant damage to the usually already injured extremity. Let's face it, you won't be putting a tourniquet on an arm or leg unless that arm or leg is already severely damaged. In that case, there's almost no way to know for sure if the tourniquet made anything worse