A shaped loaf holds its dome because the outer layer of dough has been pulled into a taut skin under tension, not because your hands pushed it into a ball. That skin resists spreading sideways under its own weight. So when gas expands in the oven, the loaf has nowhere to go but up.
Why tension decides up or out
Think of the shaped dough as a balloon skin wrapped around a mass of trapped gas and flour. Even tension around that skin holds the mass in a dome; take the tension away and gravity does what gravity does to anything soft and unsupported — it spreads. The skin doesn't come from kneading alone. It comes from dragging the dough's surface against the counter until friction catches it and pulls it smooth, the same gluten network built during mixing now stretched taut over the outside of the loaf rather than tangled through the middle of it.
Oven spring is the moment this matters most. In the first several minutes of baking, residual yeast activity peaks, trapped gas expands with heat, and alcohol boils off — all before the crust firms up enough to lock the shape in place. That burst of volume has to go somewhere. A tensioned skin blocks the easy route sideways, so the extra volume inflates the dome upward instead.
Shape a loaf with no tension in its skin and you can watch this fail in real time. It spreads into a low, wide puddle before it ever reaches the oven, and what does rise comes out coarse and uneven, because the interior had nothing to push against while it expanded.
Why pre-shape and a bench rest come first
Gluten that has just been stretched behaves more like elastic than like clay — pull it and it wants to spring back rather than hold a new position. A dough shaped straight after dividing is fighting that spring-back the whole time, which caps how tight the final skin can get. A short, covered rest between pre-shape and final shape gives that tension time to settle, so the next pull actually tightens the surface instead of just re-stretching stress that hasn't gone anywhere yet.
Degassing is redistribution, not removal
"Knock the air out" is the wrong instinct. Bulk fermentation leaves the dough full of gas, but unevenly — a handful of large, irregular bubbles rather than many small ones. Folding and tightening the dough during pre-shape and shaping doesn't expel that gas so much as break the big pockets into smaller ones and spread them through the mass. The loaf that comes out of the oven with a fine, even crumb got there because shaping redistributed bubbles, not because it emptied them.
Why the seam has to stay closed
The seam is the one place on the loaf where the tensioned skin was cut and rejoined rather than dragged smooth in one piece, which makes it the structurally weakest point by definition. Placing it face-down, against the banneton or the pan, lets the dough's own weight hold it shut through the final proof, so the tension built everywhere else on the loaf has nowhere to leak out from underneath. A seam that comes open partway through baking gives the gas an exit that isn't the top of the loaf, and the dome collapses toward whichever side it finds.
Why a slash isn't decorative
A tensioned skin under sudden pressure has to give somewhere. Left to its own devices, it picks the thinnest or weakest point on the loaf, and that's rarely the top. A slash cuts a deliberate weak spot into the skin before the loaf goes in, so the oven-spring pressure has an obvious place to go rather than an accidental one. That's the whole reason an unscored loaf tends to burst along its side instead of opening cleanly where a baker chose.