Procedure
During normal well drilling, drilling mud is pumped through the drill stem and out of the drill bit. In a drill stem test, the drill bit is removed and replaced with the DST tool and devices are inflated above and below the section to be tested. These devices are known as packers and are used to make a seal between the borehole wall and the drill pipe, isolating the region of interest. A valve is opened, reducing the pressure in the drill stem to surface pressure, causing fluid to flow out of the packed-off formation and up to the surface.
Read more about this topic: Drill Stem Test