In a junction field-effect transistor, the controlled current passes from source to drain or from drain to source. In normal usage, a voltage is applied across the channel, with the drain being made positive with respect to the source. Thus, electrons will passes through the n-type channel from source to drain. Using the voltage applied and the resistance of the channel the magnitude of current flowing through the channel can be determined. Therefore, the effective width of the channel is restricted by the depletion region, and its effective resistance is higher in this part of the channel, than in any other part of the channel.
If now the gate is forward biased with respect to the channel, the depletion region starts reducing thereby decreasing the flow of current through the channel. However, if the gate becomes reverse biased with respect to the channel, the applied electric field will increase the depletion region resulting in the narrowing of channel width, and the channel resistance is increased accordingly.
Hence, it is seen that a small voltage applied to the gate can have a profound effect on the current flowing through the channel.
If the applied reverse bias becomes very high, the channel width will become very narrow stopping the current flow completely.