The controller does the following for CDS:

     - reset array
     - delay to settle (RST_DLY)
     - integrate for DWELL 
            - read frame and transmit (null frame)
     - reset array
     - delay to settle (RST_DLY)
            - read frame and transmit
     - integrate for DWELL
            - read frame and transmit

If you didn't reset after the null you would effectively reduce the full well
of the pixel (due to the introduction of the null RD_TIME delay during
integration).  

Even with a reset after the null there is still a settling benefit from
running the clocks and video chain switches prior to the first proper CDS
frame.

..............

For CDS the DWELL is increased to give longer exposure times.
For NDSTARE, the DWELL is fixed and the last two steps in the above are
repeated to give longer exposures.