..In case something went wrong..
You are on a multi-pitch climb top belaying your partner and he can't (or won't) make it
What do you do?
This is what the second hole in the reverso-style belay device is for. Using it the belay device can be turned thereby disengaging the autoblock. It will release apruptly so you have to get more control on the breaking hand by redirecting it through a biner above the device

A nut tool is an ideal leaver. Redirect the breaking strand for control!
If you are not really really strong you will need a pulley system:

3:1 pulley, the belay device is a natural ratchet
For more leverage you have to build an additional 2:1 with the end of the rope
You are on a multi-pitch climb Top belaying, your buddy falls and is injured: