Effects - Split fieldEffects - Split fieldMasks half of the filter. Any effects following the split field will only be applied to half of the filter.
RotationThe rotation of the split. The rotation can also be set, more easily, using the dial control at the bottom.
The soft edge makes the edge between visible and invisible halves less sharp; it makes the edge softer. You can also use the filter setting "Focus" for this, but using a soft edge is considerably faster.
Click on any part of the dial and, whilst keeping the mouse
button pressed, drag it around to set the rotation angle. The dial
is marked to indicate which part will be masked off (the
transparent half) and which will remain visible (the dark
half).
The dial may "snap" to certain angles, you can configure this
through the options.