The characteristics of a JFET
When VGS &lt; 0.6 or so, a depletion region is formed between the gate and the channel (which is the conductive region between the drain and source). As VGS becomes more negative, this depletion region widens further into the channel, 'squeezing' the conductive region -- thereby reducing the current that flows.

With positive VGS (up to ~ 0.6 V or so), the depletion region narrows, and the device would run more current.

With VGS even greater than this, the gate-channel junction is forward biased, and a large gate-source current flows. This is generally not a useful mode of operation. Depending on the structure of the device, drain current may change also.