alt Hacker NewsDead-man's switch means triggering when something doesn't happen. (The name comes from a switch that an alive operator would need to hold in such a way that if they died they would stop holding.) So in this case she means that her monitoring will fire if there wasn't a successful backup within some configured period of time.
I assume this is opposed to alerting when the backup job fails, which is an issue if the job never runs, or hangs forever, or crashes in a way that doesn't trigger your monitoring.
However I don't see how any of this solves the issue of not testing your backup. Because you can definitely have a backup task succeed regularly but the thing it is backing up is still unusable.
Once hired a DBA that reworked backup scripts. He got real annoyed at the idea of testing the backups with real restores and clearly never did on the real databases (only on his smaller samples).
Turns out the backups took 30 hours. The daily backups. That then overwrote each other on the assumption that the backups would not take that long.
Of course we found out the hard way.