The system of the Rolex Grand Slam is simple: Anyone, who wins three shows in a row, receives 1 million Euros on top of the prize-money of each show. Moreover, anyone who wins two out of the three shows, also receives a bonus. If two shows are won in succession, the bonus is 500,000 Euros. If the rider is successful in qualifying for the “two out of three mode”, but doesn’t claim the victories at two consecutive shows, the bonus total 250,000 Euros. It is only the rider that counts, not the horse. It is possible to claim the bonus riding different horses. This system is infinite, it is not limited to a calendar year. It is a never ending story.
Source: http://www.rolexgrandslam.com/