Oven spring is the last burst of fermentation that occurs during the first 10 minutes of baking. Yeast responds to the extremely high temperatures of the oven by speeding up fermentation and producing a large amount of CO2 before they eventually die off. This results in a dramatic rise called oven spring. To achieve oven spring you'll need to bake your bread at the exact right moment. When it has built up enough CO2 yet has not exhausted all of its food source so that it can still support the last burst of activity needed to rise.