The retriever engages severed part of the tubing and recovers it together with the cutter. 
If, for some reason, cutoff has failed, disengage the fish by overpulling approximately 1...1.5t over retriever`s load capacity in order to shear the bolts. If, even after bolt shearing, the fish is still engaged, slack off the workstring until it freely goes down and then pull it out of hole.
When the fish is retrieved at the surface, release retriever off the fish by pushing it forward (towards the nut). Then, disconnect the retriever, clean it, grease threads, slip guideways, check slips for unconstrained motion and reliable latch of the slips to the slots. Put the tool in a tool box for the next time. 
In case of emergency retrieval, partially disassemble the tool and replace shear bolts. For that, unscrew locking screw and remove crossover, remove spring with spacers, and pull out a pusher.  
Remove heads of the the shear bolts from the pusher, and rods of the shorn bolts from cone sleeve. Install the pusher into appropriate slots of the cone sleeve, align orifices and screw new bolts in until tight. Assemble in reverse order.