What is oven spring?

The dramatic rise in the first 10 minutes of baking, as trapped fermentation gas expands, water flashes to steam, and yeast does its last burst before dying around 140°F / 60°C. Most of a loaf's final volume comes from oven spring, not from bulk fermentation. The crust has to stay pliable long enough for the rising interior to push outward — which is why steam in the oven matters so much for hearth breads. A dry oven sets the crust early; the interior expansion has nowhere to go and the loaf comes up short. Scoring also affects oven spring: a well-placed cut gives the expanding gas a designed exit so it lifts the loaf at the score rather than blowing out a weak seam.