When he had joined the other twelve versions of himself for this battle he had not intended for the day to end in this way.
Thirteen Gallifreyan centuries of his life to calculate and plan should have been enough time to identify all the variables. The thirteen iterations of the same TARDIS were equidistantly distributed in their spatial and temporal orbits of Gallifrey, the interference of the Sky Trenches had been accounted for, and the assistants had been quelled with variations of "I'll explain later".
But typically the Daleks and the Timelord General were Not Co-operating. Between the bombastic exterminating firepower of one and the pigheaded refusal of the other there had been just enough delay to begin destabilising the field the TARDISes were generating.
Each Doctor was pressing buttons and pulling levers and commanding to his TARDIS to do exactly what was expected of it as the field wavered; 3 was swearing in Venusian, 1 was stating clearly and firmly what he was doing, one of the older ones who was wearing a pin stripe suit was urging his TARDIS to 'hold on'.
The Daleks were becoming more accurate with their firepower and were beginning to target the TARDISes.
Without pausing to think of the consequences and knowing that (as per usual) it would be up to him to save the day, he chose to act and see what happened. With a flick of a switch, he locked his TARDIS's spatial co-ordinates and anchored the other 12 TARDISes. Seconds later, with a temporal 'snap', Gallifrey disappeared and the Daleks annihilated themselves.
The rebound hit 6's TARDIS hard, releasing the anchor and sending a massive wave of kinetic energy through the ship. Mel was knocked off her feet and 6 found himself being thrown across the console room with such force that he cracked the back of his skull on the hard floor.
As he felt the regeneration energy begin to seep into his cells 6 smirked and thought of the look on the Inquisitor's face when she would discover that his impulsive action saved Gallifrey.
In his semiconscious state it didn't occur to 6 that no one would remember the reason for his regeneration.
Not even himself