  We study the physical mechanism of a major X-class solar flare that occurred
in the super NOAA active region (AR) 12192 using a data-driven numerical
magnetohydrodynamic (MHD) modeling complemented with observations. With the
evolving magnetic fields observed at the solar surface as bottom boundary
input, we drive an MHD system to evolve self-consistently in correspondence
with the realistic coronal evolution. During a two-day time interval, the
modeled coronal field has been slowly stressed by the photospheric field
evolution,which gradually created a large-scale coronal current sheet, i.e., a
narrow layer with intense current, in the core of the AR. The current layer was
successively enhanced until it became so thin that a tether-cutting
reconnection between the sheared magnetic arcades was set in, which led to a
flare. The modeled reconnecting field lines and their footpoints match well the
observed hot flaring loops and the flare ribbons, respectively, suggesting that
the model has successfully "reproduced" the macroscopic magnetic process of the
flare. In particular, with simulation, we explained why this event is a
confined eruption-the consequent of the reconnection is the shared arcade
instead of a newly formed flux rope. We also found much weaker magnetic
implosion effect comparing to many other X-class flares
