If you KO Reshiram/Zekrom, it revives itself and you can battle it again and again and again until finally you catch it. If your party is full and you catch Zekrom/Reshiram, you will be asked which pokemon you want to switch with Zekrom/Reshiram if you want to switch and the switched pokemon will go straight to your PC. However, if all of your pokemon are KOd and you go straight to the poke center, there is someone on the left who takes you straight back to N's castle.
