The tourniquet referred to in knee surgery is usually an inflatable wrapping which goes around the upper thigh during surgery.
After the blood is squeezed out of the leg with an elastic bandage, the tourniquet is inflated to above the blood pressure of the patient (usually inflated to 350 mm Hg) to stop all blood flow into and out of the limb. Here is a more detailed audio explanation of the issues related to tourniquets -
Transcript of the audio file
Tourniquets and internal bleeding into the joint
One of the factors that is important to consider is whether or not a tourniquet was used during surgery, and whether or not there was great diligence in ensuring that there were no bleeders once the tourniquet was released.
A tourniquet is a band - just like the one used for taking your blood pressure - that is wrapped around the top of the leg before surgery. The blood is drained from the leg by lifting ir, or by wrapping it with a tight rubber bandage, and then the tourniquet is pumped up to well above the patient's blood pressure. This ensures that there is no bleeding during the operation.
When a joint is filled with blood the condition is known as haemarthrosis. This simply means 'blood in the joint'. A little bit of blood in the joint after injury or surgery may soon be resorbed, but when the knee is tense and painful with blood then this is very different. The blood can be aspirated and the joint washed out via a needle if it is early enough, but once the blood is clotted then only an open procedure to evacuate the clots will resolve it.