How did daniel become involved with the resistance of the camp

Daniel is so upset by the murder of the child, that he unconsciously tells his photographer friend that he used to work for the resistance while in Auschwitz. He is recruited, along with his father to help with the resistance at Buchenwald. They have a radio and know that the allies are approaching.

When the time comes, Daniel and his father are given guns and are stationed by a gate. They shoot two guards. One is killed but the other is not, and while Daniel wants to shoot him, his father tells him no, to do that would be to stoop to their level of evil and he will not be like they are. Daniel lets him live. They spend a few weeks at the camp and are careful after the allies arrive not to eat too much too fast and get sick.