What was the flashback in book Night by Elie Wiesel?

The flashback occurs when Eli (an adult) returns to Sighet, his hometown, after WWII and is arrested once again. He remembers the moment when he first arrived at the Auschwitz-Birkenau concentration camp, and in that flashback, he describes the selection process made by Dr. Mengele, deciding which prisoners were fit for labor and who would be sent directly to the gas chamber.