This feature makes the host keyboard lights match those of the virtual machine's virtual keyboard when the machine window is selected. It is currently implemented for Mac OS X and Windows hosts and available as of releases 4.2.24 and 4.3.8. The feature can be enabled using the following command:
VBoxManage setextradata "VM name" GUI/HidLedsSync "1"
In order to disable it, use the same command but change "1" to "0", or use the VBoxManage command to remove the extra data. This is a per-VM setting and it is disabled by default.