Oven spring
Oven spring is the rapid rise pastry or dough makes in its first minutes of baking, before the structure sets.
In laminated pastry, the spring is steam: water in the cold butter vaporizes and forces the layers apart. This is why puff goes into a hot oven and why a chilled pie rises higher than a room-temperature one.
If the oven is too cool, the butter melts out before the steam can lift anything, and the pastry bakes flat and greasy.