Hi Mike.
With the fuse finder, if for instance you went to do a job on a large installation with numerous fuses and DB's and the job was say to install an extra socket onto an existing circuit, to find out which circuit you need to isolate simply plug in the fuse finder to that socket then go to the relevant DB with the reciever and hold it on each fuse until it lights up/beeps telling you that this is the fuse for your particular circuit. Handy if there is no labelling of the DB.
Obviously you can use it on lighting circuits, sub-mains etc by swapping the kettle lead for the DB lead.
Its the same kind of principal with the dead circuit finder, simply plug in or attach the DB lead to the required circuit, this then sends a signal along this circuit which is picked up by the reciever at the other end there by showing you the blown fuse/tripped breaker.
Hope this helps.