The mechanism of field line reclosing is proposed as the most natural explanation of disruptive instability in tokamaks. This mechanism adequately accounts for the internal disruptive instability, assuming that only mode m = 1 develops. It is extended to the presence of two or several modes. When there is a large number of allowed modes, one can speak of free reclosing, which leads to a force-free magnetic field in a diffusion discharge. In a tokamak, B/sub Z/ much greater than B/sub theta/, free reclosing leads to a uniform distribution of the current over the column cross section and to ejection of part of the poloidal flux beyond the confines of the diaphragm. It may be stated that the disruptive instability in a tokamak is an MHD activity that flares up for a short time and is permanently present in a diffusion column. The geometry of magnetic surfaces during reclosing has been analyzed, and qualitative arguments are given to show that disruptive instability begins to develop as a result of the interaction of the mode m = 2 with the inner mode m = 1.