The internal sample (fixed volume) flowpath is used when very small volumes are required. The sample size is determined by a passage engraved on the valve rotor, allowing precise, repeatable injections.
In Position A, the sample flows through the sample passage while the mobile phase flows through to the column. The third passage is inactive. In Position B, the sample passage is in line with the column and the mobile phase injects the contents of the sample passage onto the column. The passage which was inactive in Position A allows the sample to continue flowing without interruption.