Blood-spinning is a method of providing therapeutic (and, some claim, performance) benefits to the human body through the removal, modification and reintroduction of the body's own blood. Introduced at the University of Miami's Miller School of Medicine in 1998, it involves removing a quantity of a subject's blood and then subjecting it to centrifuge (hence 'spinning'). The centrifuging process concentrates the platelets found in blood into a plasma which has a thicker constituency than normal blood. The red blood cells are usually discarded during the process.
Once the platelets have been concentrated (to a minimum of five times their normal concentration), calcium and thrombin (an enzyme) are added. This mimicks conditions that would be found in a wound, and the platelets are stimulated into coagulating into a gel. Once they do so, they begin to produce NGFs (Natural Growth Factor hormones) just as they would within the body. These NGFs are the point of the process. Once back within the body, they will serve to quickly heal injuries or accelerate the process of muscle tissue repairing itself during training. It is this latter case that causes the practice to fall afoul of WADA, the World Anti-Doping Agency based in Montreal.
The practice has caused a bit of a stir. As of June 30, 2006, several high-profile cyclists have been withdrawn from the 2006 Tour de France on the eve of the race due to their implication in a 'doping scandal' which involves blood-spinning. Although legal in other sports (FIFA allows it, for one) the Tour follows WADA rulings on the subject, and it is a no-no.
Blood-spinning should not be confused with blood doping - the practice of freezing and storing the athlete's own red blood cells for retransfusion for a simple hemoglobin and hence oxygenation boost. Although that is also illegal in some cases, there are obviously more simple medical reasons that might be done - preparation in case of injury, for example - which make that a more complex issue. The anti-doping aspect is more clear-cut in blood-spinning, as substances are in fact added to the spun blood - the calcium and thrombin. Even if those substances are found naturally in the body, the fact that they are added outside the body makes the practice more difficult to defend.
It should be pointed out that WADA does have provisions for blood-spinning to be used as a strictly medicinal procedure - waivers can be granted for athletes who have been injured and who are utilizing the procedure for restorative purposes. It has been shown in some cases to heal tissue faster and more smoothly than mechanical aids (stitching, etc.) and as such is highly useful for contact sports such as football. The resultant gel can be injected into deep tissue injuries such as tendon and ligament tears with less problem than artificial aids, since it is composed of the body's own substance, and accelerates healing. The WADA first ran up against blood-spinning due to the UK football team Chelsea's physician Bryan English utilizing the technique beginning in 2003 for rehab purposes, which resulted in the waiver process being set up.
The problem is that the gel can presumably be injected back into the athlete after being rediluted, perhaps in more of the athlete's own blood. This would give a boost in NGF levels, which would offer an advantage in training by (I am assuming, as I am not a doctor) boosting muscle mass.
Sources:
http://www.sportinglife.com/football/news/story_get.dor?STORY_NAME=soccer/05/03/31/SOCCER_Blood.html
http://www.newscientist.com/article/mg18624994.800.html
http://football.guardian.co.uk/News_Story/0,1563,1514185,00.html
http://sportsillustrated.cnn.com/2006/more/specials/tour_de_france/2006/06/30/bc.cyc.tourdefrance.dop.ap/index.html