The Bitplane filters treat the image as a series of planes. Each RGB component of each pixel consists of eight bits - bits 0 - 7. The Bitplane Filters treat all the corresponding bits of all the pixels as a single plane. So for example, all the bits which are in position zero in all the pixels are treated together as plane 0.
These filters work by removing or moving individual planes in the picture.
We can take a picture and see how each plane contributes to the whole picture:
|Plane 7||Plane 6||Plane 5||Plane 4||Plane 3||Plane 2||Plane 1||Plane 0|
As you can see, the major contribution to the picture is from the higher planes, while all the detail is from the lower planes. This means that progressively cutting planes from the lower end removes detail and so gives a more and more posterised effect:
While removing the higher planes gives a more and more random colour effect: