An article in the Guardian today draws attention to the practice seen in some US athletes at the olympics of "cupping"
The article quite rightly points out that the practice has no scientific basis, nor any evidence of efficacy, and that any benefit perceived by the athletes is likely to be pure placebo.
However, it is just possible, maybe, that there may, theoretically, be some real physiological effect.
"Oxygen flux" is the amount of oxygen delivered to the peripheral tissues per minute. Assuming no pathological physical impairment of blood flow it is very much dependent on haemoglobin value. The higher the Hb, the more oxygen the blood can carry, and the greater the oxygen flux. Up to a point....... But increasing Hb also increases blood viscosity and thus reduces flow. There comes a point when increasing the Hb causes such an increase in viscosity that the resultant reduction in flow completely negates the effect of increased oxygen capacity, and further increase becomes counter productive. This is well known to happen in polycythaemia. So there is an optimal value of Hb at which O2 flux is maximal. The exact value comes as a surprise to many doctors. Optimal Hb for maximal O2 flux is (in old units as befits DZ) just 10g/100ml, significantly lower than the normal 14 or so.
If the bruising caused by cupping were to significantly lower the Hb level in the blood, it would improve peripheral blood flow, and possibly, improve performance. You might get a similar effect by bleeding, or applying leeches.
As this study shows, removing 500ml blood certainly doesn't seem to have a negative effect. And as far as I'm aware, unlike performance enhancing drugs, it's not forbidden.