Trigger mittens are a hybrid design that keeps your fingers together in a single compartment for warmth, but separates your index finger into its own pocket so you can grip your ski poles properly. Think of them as mittens with one finger free — they give you near-mitten warmth while still letting you hold poles and make quick adjustments without taking them off.
You'd choose trigger mittens when you're skiing in genuinely cold conditions but still need reliable pole control. They're popular with racers and anyone whose hands run cold but doesn't want to give up pole grip. Compared to 3-finger (lobster) designs, trigger mittens are warmer since more fingers share a compartment, but they offer less overall dexterity — you're really only getting that one free finger for grip.
If you've been struggling with cold hands in regular gloves but find full mittens too clumsy for pole use, trigger mittens are worth a look.