The idea is that Stalin decides that it is not in his interest to create an ethnically more heterogenous Poland by moving Poles from areas that OTL what taken from Poland and given to the Soviet Union to areas OTL taken from Germany and given to Poland. Instead he decides that the former German areas shall become part of the Soviet Union and become inhabited by (mostly Russians, but also Belorussians, Ukrainians and others) people from the Soviet Union. Some Germans and Poles might perhaps be allowed to stay there, but most of the population would come from the Soviet Union.