I think a trigger point is more of an attribute to a particular fly, that is not meant to resemble an attribute of the natural explicitly. Such as adding a strip of flashabou flashback to a wing-case of an otherwise perfectly impressionistic pheasant tail nymph. Or a new dubbing such as Ice-dub to an existing fly adding flash. Or adding a colored bead head to a nymph that is not a normal color within the ecosystem. When these types of additions work and cause fish to take the fly noticeably over the closer to natural pattern, most label that flash or color as a "trigger point".