  A transport barrier in a toroidal plasma is treated as the boundary region
between the hot and cold phases of the plasma. These two phases possess
essentially different collisional and transport properties, so the boundary
region between them corresponds to the minimum values of specific resistivity
and transport coefficients of the plasma. The behavior of magnetic shear in the
region of a transport barrier and some other specific properties of transport
barriers are the consequences of the minimum plasma resistivity in this region.
The origin of sawtooth oscillations in toroidal plasmas is shown to have a
similar nature.
