Double-loop fire detection systems can either be the capacitance/resistance type, or the gas filled detector type. They are inherently more reliable than single indication fire detection systems, as the loops run together, and if both of them indicate a fire, the indication is much more likely to be correct. They operate using an AND logic gate, so both have to indicate a fire to be taken as a fire warning. This avoids quite common false positive indications that could happen on single loop systems.

If a loop is crushed during operation, then - depending very slightly on the type of loop - the system will likely register a fire/overheat/error on that loop, but that makes no difference as the other loop (Loop B) is not registering a fire. Therefore, there will be no indication at the time (not even a fault indication), and the system automatically acts as a single loop system, based on whether loop B detects a fire or not. That is what the correct answer means about the system "operating as normal", but we would say it's not exactly normal, as it's a single loop system now.

Depending on the MEL (Minimum Equipment List) of the aircraft/operator, flights can be undertaken with one loop broken before flight, as the system can change its logic from requiring both indications to just one (it can run as a single loop system). This is effectively the same as if a loop fails in flight, and does obviously reduce the reliability of the system though, so the crew must be careful in doing this.