You are checking the trailer emergency brakes. In which position should the tractor protection control valve be placed?

Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called tractor protection valve control or trailer emergency valve), or place it in the "emergency" position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on. [California Commercial Driver Handbook]