Well, I'm not sure about the transistor's damage ,but the basic idea pf the J-FET transistor is modulating the current passing from drain to source by the applied voltage to the gate (in fact, this is the idea of almost all transistors) .This is preformed by variation of the channel's width through which the current passes form drain to source by the depletion region of the reversed PN junction due to the gate voltage (depletion width is proportional to root the voltage) .Ideally, it's required that no current passes from source or drain to the gate .When you apply enough positive voltage to the gate to make the junction forward biased, current can pass directly from the source (drain) to the gate and gate voltage will no longer control the drain-source current ,thus, the J-FET won't function as a transistor .