This button toggles the keyboard focus capture mode. In certain hosts there's a problem - when you click on the plug-in window the keystrokes are no longer sent to the host. As a result keyboard shortcuts (such as spacebar f.ex.) no longer work. This button prevents from capturing focus. The side effect, due to the nature of windows message handling, is that the mousewheel messages won't go the plug-in either. If your host does not exhibit such focus problems (stealing spacebar) don't touch this button.