In the fuel tap is a low-pressure valve that will open, when the engine is still running.
If the engine is off, the valve is closed like "Off" Positions from different fuel taps.
The "PRI" function goes around these valve and floods the floatchamber without the running engine.
Normaly these flooding will stop from the valve needle.
On these picture it is #33
http://alpha-sports.com/spst/1995%20GS500E/06.gif