Each coil in a humbucker is smaller, and often features fewer winds than an actual single coil, and this translates to it's sound. Many find the 'split' sound of a humbucker to be quite shrill and thin, when compared to the humbucking sound, or a traditional single coil pickup. Certain modders and manufacturers have devised a way to overcome this, that being the 'partial' coil split. By utilising a resistor to send the second coil to ground, rather than sending the wires directly to ground, you allow yourself to essentially lower the volume of one of the coils, as opposed to shutting it off completely. This results in a thinner and quackier tone than the full humbucker, but a more full and warm sound than the split coil, and is often a far more musical and useable tone for many guitars.
This mod is simple to add to any guitar with a pre existing coil split, simply install a resistor (5.6KOhm is my preference) on the 2 way on/on DPDT or push/pull pot. Remove the ground wire on the lugs and replace is with your resistor going from the lug to ground. Simple!
My prefered way to partially split is to use a 3 way on/off/on DPDT, in the middle lug you will connect your coil split leads from the humbucker, on one side of the switch you will send the lugs directly to ground, and on the other side you will send the lugs to ground through the 5.6KOhm Resistor. Meaning that you now have 3 unique tones: Humbucker, Partial Split Humbucker, and Single coil.