If a Time Lord has kept a part of his or her body that had been removed (like the 10th Doctor's hand, from before), it can be used to keep a Time Lord from changing during regeneration. The 10th incarnation of the Doctor kept the hand that was cut off in the sword fight in a jar on board the TARDIS. Eventually, he was shot by a Dalek, and his body began the regeneration process. After his body had finished healing, but before his body could change into a new one, the Doctor sent all of the extra energy into the hand in the jar.