Just for some clarification........
Opening the cylinder valve slowly.......this is for the 'excess flow valve' inside the big green ACME nut on the hose. It is a ball bearing that moves towards close when valving in a cylinder. When the downstream pressure builds up the pressure equalizes and allows the ball bearing to move back towards center. If you valve in the cylinder to quickly the ball bearing can fully close stopping flow.
It is a safety device...should a line/fitting break downstream (excess flow) the ball bearing closes stopping excess propane flow.
The 'OPD'.......overfill protection device. Basically a float that closes when liquid level reaches 80% capacity. Sometimes the float can stick and 'banging cylinder' on ground will free the float allowing propane to flow.
The cylinder also has a spring loaded valve in hose connection. IF a hose is not attached to cylinder the spring loaded valve is closed...no propane can flow.
Also the ACME nut has a 'thermal bushing' that will melt in a fire and then the cylinder spring loaded valve will not be held open anymore....no propane flow.