When a pressurized gas is released, the velocity of the flow will heavily depend on the pressure difference between stagnant pressure and downstream pressure. By assuming an isentropic expansion of an ideal gas from its stagnant conditions (P , meaning the velocity of the gas is zero) to downstream conditions (P, positioned at the exit plane of the nozzle or orifice), the subsonic flow rate of the source term is given by Ramskill's formulation:    
As the ratio between downstream condition pressure and stagnant condition pressure decreases, the flow rate of the ideal gas will increase. This behavior will continue until a critical value is reached (in air, P/P is roughly 0.528, dependent on the heat capacity ratio, γ), changing the condition of the jet from a non-choked flow to a choked flow. This will lead to the a newly defined expression for the aforementioned pressure ratio and, sub-sequentially, the flow rate equation. 
The critical value for the pressure ratio is defined as:
This newly defined ratio can then be used to determine the flow rate for a sonic choked flow:
The flow rate equation for a choked flow will have a fixed velocity, which is the speed of sound of the medium, where the Mach number is equals to 1:
It is important to note that if P keeps on decreasing, no flow rate change will occur if the ratio is already below the critical value, unless P also changes (also assuming that the orifice/nozzle exit area and upstream temperature stay the same).