Used in Ambulances and Hospitals. In cases of shock, the EMT slips these onto a patient. They work by inflating the pants like 2 large blood pressure cuffs, pushing the blood toward the brain.
Used more rarely, because they are out of protocol for most EMTs. They can't be used in cases of a suspected leg fracture.
What is reccomended more instead is tilting the patient's stretcher/backboard 30 degrees elevated with the head lowered to pool the blood. Also known as the Trendelenberg position.