An elastic section in the chest strap is a stretchy segment that gives slightly as you breathe deeply. Instead of the strap holding rigidly across your chest, it expands and contracts with your breathing, which makes a real difference when you're working hard on the skin track.
This feature really shines during high-output backcountry touring. When you're climbing uphill and breathing heavily, a rigid chest strap can feel restrictive and uncomfortable over time. The elastic lets your chest expand fully without having to constantly loosen or adjust the strap. For resort skiers or casual sidecountry laps, it's less critical since you're not typically skinning for hours at a stretch.
If you spend a lot of time on human-powered ascents, an elastic chest strap is a small but worthwhile comfort upgrade. If you mostly ride lifts, you probably won't miss it.