They are not "used" as combustion sensors, rather they simply sense what they normally sense and are relayed back to the coils which have the technology and sensing ability. The plugs are just like any old plugs.
The reason they are important is because if they have carbon built up or residue than the values can be off and thus cannot properly manage knock and timing targets. It ends up cutting a lot of timing with old plugs given it masks the signal
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