Inner thigh vents are zippered openings along the inside of each leg that let you quickly dump heat when you're working hard on the mountain. They're the most common vent placement on snow pants because your thighs generate a lot of heat during active riding, making this spot particularly effective at cooling you down fast.
The big advantage of inner thigh vents is convenience—you can open and close them while riding without stopping, which is perfect for resort days where you heat up on a long traverse but cool off on the chairlift. They're a proven, reliable design that works well for almost every riding style.
If you mostly ride at the resort and want solid temperature control without the added cost and weight of full side zips, inner thigh vents are the way to go. Consider mesh-lined versions if you ride deep powder, since the mesh keeps snow from sneaking in while still letting air flow.