safety - Self Checking Circuit

I was reading the specs. of an electronic module used in process-safety applications & came across this note:

Self-Checking (Circuitry): a circuit with the capability to electronically verify that all of its own critical circuit components, along with their redundant backups, are operating properly. Banner safety light screen systems and safety modules are selfchecking.

I'm curious, how exactly does one design a self checking circuit? Can someone post a circuit diagram of a self checking circuit?

Does the verifier have to only verify the primary or does the verifier circuit verify the verifier itself?

Edit: Here is a link to the actual device, a "Banner Muting Module ": http://info.bannerengineering.com/cs/groups/public/documents/literature/116390.pdf

A breif description of this device's function:

The Banner MMD-TA-11B / MMD-TA-12B Muting Module (the Module) is an accessory component of a safeguarding system, which may incorporate such primary safeguards as safety light screens, safety interlocked gates/guards, or other presencesensing safeguarding devices (PSSDs). The Module allows the machine to mute the primary safeguard by monitoring redundant inputs (two or four) and automatically suspend the safeguarding function of a safeguarding device during the non-hazardous portion of the machine cycle.