The transducer is brought to full scale differential and the piston is simply held in position. If either side of the system leaks, the output will decrease.
Leak rate will be reported in inches of WC/sec. If there is a very slow leak, the pressure controller will automatically compensate for leak rates up to 0.0001" WC/sec. This test is designed to determine if the system is sealed properly, not just to leak test the transducer.
For more specific information on leak testing, please refer to the operating instructions here.