What did the prisoners have to do when they heard "selection" and what was the process?

In chapter six, selection is called. Those deemed healthy are kept and those deemed worthless are sent to the crematory. On the third day there is a selection, and although Eliezer's father is sent to the crematory group, Eliezer creates a disturbance so that he manages to sneak back into the other group. The prisoners wait standing for a train in the middle of a snow-covered field, and because they are deprived of water and forbidden from bending over, they begin eating snow from each other's backs using spoons. The SS officers simply laugh. Finally, a train arrives, and they are loaded in, a hundred per car.