Originally Posted by
WeirdBits
Out of phase or split? The two wires connected together are usually the series link combining the two coils of a rail or humbucker, so shorting that to ground or output with a switch allows you to split to a specific coil. Which coil you want to split to depends on the brand of pickup and potentially what other pickup you're wanting to combine it with.
Changing the phase usually refers to reversing the connections of a pickup (or coil) when combining it with another pickup (or coil) to get that nasally out of phase sound, and often happens accidentally when usually two different types/brands of pickups in a build.
Need a bit more info about what you're trying to do etc. before I can give you a more specific answer.