What was Bela Katz forced to do once he was chosen to work in the crematory because of his strength?

Once the Jews arrive at Birkenau, the reception center for Auschwitz, there is a period of "selection." Obviously many of the Jews, especially the old, weak and sick, were selected for immediate extermination in the concentration camp's gas chambers and crematories. Elie's mother and one of his sisters were probably chosen for death at this initial selection. Elie and his father, however, are selected for slave labor. During this period the SS officers were also sorting out the prisoners to identify the most able among them. Those that were determined to be the strongest joined the "Sonder-Kommando" and went to work in the crematories. One such man was Bela Katz, who came from Elie's town. He was the son of a tradesman and had arrived with the first group of Jews from Sighet. When Bela found out that Elie and his father had arrived he got word to them as to his whereabouts and the awful news that he had actually had to put his own father into the oven of the crematory.