although the channel is said to be in pinch off there is some channel left even though very high voltages are applied.... the main thing is that the current density is very high under pinch-off.......
In pinch off , i hope u know how the channel ll be , ..the channel ll be tapered in one end (drain) and increases linearly till source.. but, still u ll get max Id flowing through it.. u can imagine, how the water flows out of tube, when u press the hole partially... Hop u get the meaning ..
when the vds increases above vd(sat) the channel will move towards the source end..this is called channel length modulation.....this happens after the pinch off occurs.....
after the pinchoff...... the extra vds which is applied in the drain...... creates the electric field around the drain............as a result of this electric field .............the electrons coming towards the drain will be accelerated into the drain......... thus there will be current flowing even after the pinch off occurs..........
hope this answer is understandable.......see sedra smith for further clarification.........