Make sure you've got some form of lock screen security set up through the Settings > Security menu. (androidcentral.com)
Should you move out of range and drop a connection with one of these «trusted» devices, your tablet will activate its normal lock screen security once again. (phandroid.com)
First off, you need to have some sort of lock screen security on your connected phone in order for this process to work. (androidcentral.com)