Two- and three-dimensional resistive MHD simulations are used to study the behavior of a dynamic magnetotail model when reconnection proceeds from closed to open field lines. For this purpose, the initial, quasi-static magnetotail configuration is set up for various lobe density values. Initiated by the increase or occurrence of anomalous resistivity, all simulations then exhibit the growth of an unstable tail mode, which leads to neutral line and plasmoid formation. The reconnection process starts in a plasma sheet region of high mass density and progresses to open field lines with much lower plasma density and much lower plasma &bgr;. At the time of transition, drastic changes of typical dynamical parameters occur, involving strongly enhanced electric fields and reconnection rates and the generation of field-aligned currents. Further, typical substorm signatures such as the dipolarization of the magnetic field in the inner regions of the magnetotail appear on very short timescales, comparable to those found in spacecraft observations. It is argued that this behavior should indeed become important during substorm evolution, in support of substorm models based on this transition. ¿ American Geophysical Union 1996