This question already has an answer here:
Yeasts continue producing CO2 until they die at 60C/140F. Also, gasses expand with heat, so it will also help holes to grow a bit (if I remember well, up to 30%). But that grown will stop when dough gets baked and strengthens, and when crust begins to form. To retard this 2 tricks are used:
Use steam in the oven the fist 1/3 or 1/4 of baking time. Steam will keep the "outer skin" of the bread humid, so it will prevent it from getting dry and forming the crust.
How to use steam in the oven for getting an oven spring?
Well, by steam the only thing I understand is the "vapors". How do you get them in oven? By boiling uncovered water in the oven? How much steam is needed? What is the exact procedure to get the required steam?