Continuing the discussion from SGP Imaging Run Complete (But Not):
You can read about it in detail in the PHD2 help file.
In a nutshell, run the Guiding Assistant (Tools => Guiding Assistant) and leave the Measure Declination Backlash box checked. Make sure you choose a star near the center of the field since the backlash measurement will require a very large movement of the star. If you want you can skip the guiding assistant measurement phase (click Stop) and let it proceed to the backlash measurement phase. When backlash measurement is complete, there will be an option to enable backlash compensation, and you can click “Apply” to enable it. You can disable it or change the compensation amount later in the brain.
When declination guide pulses reverse direction, a backlash compensation pulse is applied to take up the backlash. The difference between PHD2’s BLC and the BLC in your mount is that when PHD2 manages BLC it can detect overshoots and undershoots and dynamically adjust the BLC pulse size, and that is what we do. (Enabling BLC in the mount is strongly discouraged since it can case overshoots and oscillation.)
As mentioned earlier, make sure you are using 2.6.1dev3 or newer for best results with BLC.