"J" valves are similar to "K" valves. What makes the "J" valve unique is a reserve mechanism. This is a spring-loaded valve that closes airflow when the cylinder's pressure drops below about 500 psi. The reserve is then opened by pulling a rod that's attached to the reserve lever.

"J" valves are no longer considered to be an acceptable method of monitoring your air supply because reliable cylinder pressure gauges have become standard equipment. For this reason, if you use a cylinder that's equipped with a "J" valve, it's important that you disable the reserve mechanism to prevent your air supply from being shut off while you still have air remaining. To disable the reserve mechanism, turn the reserve lever to the down position prior to your dive.