Valve which limits pressure to a safe level by opening fully when it opens rather than opening to an amount proportional to the flow and pressure. Pressure that is high enough to raise the primary disc off of the valve seat, then reaches the larger second disc, forcing it fully open, relieving pressure through the larger opening. Reseating of the vale is at a predetermined value.
A running back or receiver that the quarterback will look to pass to if all other receivers are covered. Usually the safety valve will not be too far away from the quarterback. Passing distance will be minimal, but avoids a sack.
A safety valve is a valve mechanism for the automatic release of a gas from a boiler, pressure vessel, or other system when the pressure or temperature exceeds preset limits. They are often called by more specific names such as pressure relief valves, T&P valves, or temperature and pressure relief valves.
usually (but not necessarily) a running back sneaking out to the flat or running a Zero pattern usually late (after initially blocking) to provide an emergency release point for the quarterback in the event of a significant pass rush.