When Bleach chapters are collected into volumes, a new chapter always starts on the left hand-side page. This leaves the right hand-side page blank. As such, Tite Kubo often uses the right hand-side page to provide a small sketch. There are four types of sketches.
Sketch Type 1
The first type of sketch shows a scene immediately following the events covered by the chapter that precedes it. An example of this is the sketch that followed Chapter 1 of the manga. When collected in Bleach Volume 1, the sketch page showed the then unknown figure of Kisuke Urahara approaching the wounded Ichigo and the powerless Rukia.
Sketch Type 2
The second type of sketch provides additional information that complement the story that has just been told. An example of this is a sketch that followed Chapter 4 of the manga in Volume 1 that gave the layout of the apartment complex Orihime Inoue lived in at the time and the location of her apartment.
Sketch Type 3
The third type of the volume sketch provides symbolic pictures that relate to the events that unfolded in the chapter the sketch follows. These sketches often have dark overtones and usually follow events that general do not cater to the levity of volume sketches in-general. An example of this type of sketch is when a shadowy figure with a reaper's scythe was drawn following the release of Kazeshini for the first time in volume 38.
Sketch Type 4
The last type of volume sketch actually tells a story unrelated to events unfolding in the chapters, but that may be taking place chronologically in tandem. These sketches can often serve as a prologue to a special omake chapter or culminate into a full colour spread. An example of this type of sketches are the sketches that populated volumes 22 and 23. Collectively, these sketches told the story of Kon's attempt to romantically conquer Soul Society, the later few sketches also served as a lead-in to a short Omake chapter concerning the Shinigami Women's Association.