Information provided by Wikipedia:
The rear flank downdraft or RFD is a region of dry air wrapping around the back of a mesocyclone in a supercell thunderstorm. These areas of descending air are thought to be essential in the production of many supercellular tornadoes. Large hail within the rear flank downdraft often shows up brightly as a hook on weather radar images, producing the characteristic hook echo, which often indicates the presence of a tornado.
Rear-flank downdrafts have a well-established association with hook echoes. Firstly, the initial rear flank downdraft is air from aloft transported down to the surface by colliding and mixing with the storm. Secondly, hook echoes form through advection of precipitation from the rear of the main echo around the region of strong updraft. Thus, precipitation loading and evaporation cooling induced by the hook echo can enhance the downdraft. Some observations showed the presence of an enhanced downdraft in the vicinity of the strongest low-level rotation, behind the main storm updraft. Further, it was concluded that the presence of a hook echo can reflect downdraft intensification.
It has been realized by many researchers that rear flank downdrafts, especially those associated with hook echoes, are fundamentally critical to tornado formation. Fujita (1975) initiated the Recycling Hypothesis: Firstly, downdraft air is recirculated into the (developing) tornado, which results in an appreciable convergence on the back side of the (still developing) tornado. Then the downward transport of the angular momentum by precipitation, and the recycling of air into the tornado, will create a tangential acceleration required for the intensification of the tornado.
The drag force from the droplets initiates circulation in an RFD. Dry environmental air is also entrained into the downdraft and evaporative cooling helps create more negatively buoyant air. As precipitation falls and cool entrained air circulated downward and eventually reaching the surface. This circulation contributes to the circulation to form a hook echo.
So essentially when a thunderstorm reaches its mature stage, updrafts and downdrafts exist side by side with a slight tilt to allow for air flow. The energy involved with the downdraft is essential in fueling the updraft which will then suction up the rotating funnel of air on the ground causing a tornado to form.
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