I have heard several writers assert these are meaningless and illusory.
Today I came upon an interesting use of the term in Law, and nothing to do with firearms.
There is a lot of debate in western society about choice over life and death for those who are 'terminally' ill. There was a recent court case in NZ where as in some European countries, especially Netherlands and Switzerland where the law has been relaxed somewhat, there is debate in changing/ shifting attitudes and a move away from the 'guilt' associated with suicide, or assisting suicide, or assisting choice of end of life.
The Court recently referred to 'trigger' as 'minimum requirement' (say) that must be met in order to satisfy a case in defense or prosecution, in other words to initiate a further set of outcomes/ procedures/ treatment.
Applied to roulette this concept is a device, possibly one of many, that enables the player to steer through the murky oncoming stream of live events/ experiences being faced at the table, providing a possible signpost based on prior empirical knowledge ( live result experience from vast testing). Encountering a familiar 'threshold'. ie trigger, the direction/ course being steered can be set with more confidence than a rough guess. The rough guess is of course not the same as an intuitive guess for that can sometimes be inspired, being based on access to vast experience but wisdom from another (higher) level. Such activity is 'magic' to the ignorant, and is subjective, but in order to be valued, must be successful, for after all, we do make our own luck (at least at our best)