What is the Document Lock Plugin?
Retrofits aspects of lock-based collaboration onto SBS documents, with an emphasis on lock visibility/awareness over absolute enforcement.   It is very difficult to enforce a persisted (non-session-based) Lock, like a Source-Control system, as a plugin given the current state of SBS.   Instead this plugin focuses on the ability to offer a social process to help people coordinate better when collaborating on Documents.

For more information check out the following links:
Document Lock - Plugin
Document Lock - Video Walk-Through