Most monitoring watches for something bad to appear: a 500, a timeout, an expired certificate, a slow response. A heartbeat monitor does the opposite. It watches for something good to stop appearing .
Source: [Dev.to](https://dev.to/merlonix/a-dead-mans-switch-that-pages-once-and-goes-quiet-is-worse-than-none-ours-went-silent-for-43-days-1f0n)