The first dorsal fin carries a stout leading spine. Behind it sits a much smaller second spine. When the fish erects the first spine, the second slides forward at its base and holds it upright; the arrangement acts like a latch, and the large spine cannot be folded down by external force while the small one is engaged. Depressing the second spine, the trigger, releases the first.
A threatened triggerfish reverses into a reef crevice, erects and locks the spine against the roof of the hole, and often flares the pelvic girdle downward as well. The result is a body wedged in place. A predator pulling on the exposed tail cannot extract the fish without breaking either the fish or the rock.
The same mechanism makes handling awkward in fisheries and aquaria, and it is why triggerfish are difficult to remove from nets and traps. The name is descriptive of the anatomy rather than of any behaviour toward humans, a point often muddled with the separate reputation of the titan triggerfish for biting divers.