A tourniquet in knee surgery is an inflatable wrapping which goes around the upper thigh to stop blood flow during an operation.

Before inflating the tourniquet, the leg is usually elevated and wrapped toghtly with an elastic bandage to 'exsanguinate' it (get the blood out). Then the tourniquet is blown up above the level of the blood pressure, and the elastic bandage is removed.
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FORUM DISCUSSIONS
Bruising and pain after tourniquet.
CLINICAL PUBLICATIONS
Tourniquets and exsanguinators: a potential source of infection in the orthopedic operating theater? Brennan SA, Walls RJ, Smyth E, Al Mulla T and O'Byrne JM. Acta Orthop. 2009 Apr 29; 80(2): 251–255.