Buerger's test is performed in an assessment of arterial sufficiency.
(This part of the test checks for elevation pallor.)
[1][2] From a sitting position, in normal circulation, the foot will quickly return to a pink colour.
Where there is peripheral artery disease the leg will revert to the pink colour more slowly than normal and also pass through the normal pinkness to a red-range colouring (rubor - redness) often known as sunset foot.
This is due to the dilatation of the arterioles in an attempt to rid the metabolic waste that has built up in a reactive hyperaemia.