A fireman's belay is a very common safety technique where a canyoneer at the bottom of the rappel pulls hard on the rappel rope if the canyoneer on rappel starts to fall.
To provide a fireman's belay, a canyoneer at the bottom of a rappel simply holds the rappel rope, perhaps wrapping it around his hand, and watches the canyoneer on rappel. If the canyoneer on rappel starts to fall, the belayer pulls hard and constantly on the rappel rope, which acts the same way as if the rappeller had pulled down with their brake hand. For long rappels, less static rope, and if the belayer is slow to respond, slowing the rappeller's descent may take several seconds, so the belayer should maintain as much force on the rope as he can until the rappeller stops descending.
A fireman's belay can be provided to a rappeller at almost any time without affecting the rappel. The belayer must simply make sure he is not applying any force to the rope unless the rappeller falls. One safety consider, however, is that the rappeller may kick down rocks and providing a fireman's belay may be dangerous for the belayer in certain situations.