A null reaction is a reaction with a catalyst or sparker that is equivalent to if the catalyst or sparker was not there. The term was popularised and possibly created by David Raucci, although he incorrectly believed the term was already in common use at the time.
This is a period-18 sparker. The first interaction with the two period-6 sparkers is required for the oscillator to function, but the second is a null reaction — some of the sparks evolve differently, but it doesn't affect the overall evolution.