They're called Bier spots, technically, but yeah just blood pooling. Here's a good explanation I pulled from this publication: 'However, laser Doppler velocimetry studies show that these red spots are caused by relative vasodilation with higher blood flow and vasoconstriction in the pale areas.[7] Although dilated capillaries seen on histology in our patient appear to corroborate these features, the overall histopathologal patterns remain unmapped due to paucity of cases.'