With two humbuckers connected by a switch in phase pick ups will sound normal, while out of phase humbuckers will sound weak and nasal. This is because out of phase humbuckers result in a lot of low and mid frequency cancellation. With Strat and single coil type guitars being in phase is the problem and if adjacent pickups are in phase the combined selection will not be noise canceling.
This may be a confusing explanation. What we need to emphasize here is that humbuckers work because the coils are out-of-phase and the magnetic polarities are opposite. Either condition by itself will produce conflicting (out-of-phase) string signals from each coil that will result in a thin, nasal sound, but the two conditions together produce a full and powerful string signal and cancellation of most of the hum.