Opening the relief valve allows water to flow thru it helping to remove scale/crud that can build up on valve seat----especially if water supply is still on.
Just stand back and let it blow.
Recommended to preform at least once a year (RV and Resdiential WHs)
ANNUAL OPERATION OF T&P RELIEF VALVES:
WARNING: Following installation, the valve lever MUST be operated AT LEAST ONCE A YEAR by the water heater owner to ensure that waterways are clear. Certain naturally occurring mineral deposits may adhere to the valve, blocking waterways, rendering it inoperative. When the lever is operated, hot water will discharge if the waterways are clear. PRECAUTIONS MUST BE TAKEN TO AVOID PERSONAL INJURY FROM CONTACT WITH HOT WATER AND TO AVOID PROPERTY DAMAGE.
GO on...do it. Lift the lever and let it flow