You are referring to a safety device to prevent catastrophic leak events, It consists of a small ball that sits in a chamber. With normal LPG flow the gas goes around the ball and works normally. With a big rush/flow of gas the ball will move and block the orifice allowing only a tiny amount of gas to pass.
After triggered, the ball will continue to block the orifice due to the LPG pressure holding it in place. To reset the safety device, the cylinder valve needs closed and pressure in the system reduced. The ball then drops into its normal position and will allow normal gas flow once again.
Do you all also know that the portable LPG bottles are correctly called a DOT cylinder? An LPG ASME 'tank' is permanently mounted like in a motorhome.
Shopping for a fiberglass TT
2009 Pontiac Vibe pulling a 2009 Aliner Sport = 22 MPG
1998 Ford E150 pulling a 2006 Weekend Warrior toy hauler = 8 MPG