When the injected sample first goes onto the head of the capillary column, the column is flooded. This is because the column diameter is small in comparison to the amount of sample that is injected. About 6 inches to a foot of the head of the capillary column is in the injector port and the remainder of the column is in the GC oven. The head of the column is typically at a much higher temperature than the rest of the column. Solvent focusing happens at that interface between where the sample is injected and where the sample starts its transit down the column.