The current standard for mid/high-level markers is to include a system of electronic detection for determining if the next round is loaded or not loaded. The purpose of this is to prevent the chopping of the next round due to the ball being half-loaded when the firing commenses. This page was created to explain how the "eye" system works, and its applications.
I'm currently re-writing the article with new and expanded information; it was originally written in 2004 and is horribly outdated and disorganized. Until the new article is available, you can find my two new sub-articles using these links:
Electronics theory - Eye components and hardware design.
Detection methods - Eye detection concepts and associated circuit programming.