A wide yoga-style waistband is a flat, broad band that distributes pressure evenly across your midsection instead of digging in like a traditional elastic band. It stays put and won't roll down, which makes a real difference when you're bending, twisting, and sitting on chairlifts all day.
The big advantage under ski gear is that it lays completely flat under your ski pants and mid-layers. No pinching, no visible ridge, no uncomfortable pressure point where your base layer waistband meets your snow pants waistband and belt. It's especially nice for anyone who's had base layers dig in or shift around during a full day on the slopes.
This style is most common in women's base layers, but anyone who prioritizes comfort under layers will appreciate it. If you've ever ended a ski day with a red mark around your waist, the yoga-style band is worth seeking out.