The sample loop is loaded in Position A. When the valve is switched, the contents of the loop are injected onto the analytical column. The front cut elutes from the column and goes to vent at port 5. The valve is then switched back to Position A, and the heart cut is backflushed to the detector). Once the heart cut has eluted, the valve is switched back to Position B, sending the end cut to waste.
By delaying the switching times, the front and heart cuts could go to waste, with the end cut going to the detector.