[MRM]

It's a hair trigger used for precision shooting. The first trigger has normal trigger pull so you squeeeze it slowly to keep the gun aimed on target. That's normal shooting. When you're shooting on a bench or otherwise braced and you'e shooting for tight precision, you use the hair trigger. The pressure to trip the hair trigger is probably a few ounces, as compared to a few pounds for the regular trigger, so the lightest touch fires the gun. You have less movement than squeezing the regular trigger so it's easier to keep the gun still as you fire and you get better precision in hitting your target with a tight group.