Partial Eruption, Confinement, and Twist Buildup and Release of a Double-decker Filament

We investigate the failed partial eruption of a filament system in NOAA AR 12104 on 2014 July 5, using multiwavelength EUV, magnetogram, and H$\alpha$ observations, as well magnetic field modeling. The consists two almost co-spatial segments with different end points, both resembling C shape. Following an ejection precursor flare related to flux cancellation, only upper segment rises then displays prominent twisted structure, while rolling over toward its footpoints. lower remains undisturbed, indicating that possesses double-decker structure. erupted ends up reverse-C shape, material draining footpoints, losing twist. Using rope insertion method, we construct model source region qualitatively reproduces key elements observed evolution. At onset, atop bundle negligible twist, which is consistent observational interpretation reaches critical height torus instability during initial relaxation, stable equilibrium. terminates when dome-shaped quasi-separatrix layer reminiscent fan surface, although no null found. destroyed by reconnection confining overlying above dome, transferring twist process.