When we make yeasted breads such as Challah, we press the dough gently with our knuckle or finger to determine if it is properly proofed and ready for baking. If the dough springs back right away, it needs more proofing. But if it springs back slowly and leaves a small indent, it's ready to bake.
Here's why the test works: The dough is essentially a collection of air bubbles contained by a network of gluten. As yeast consumes sugars and generates gases, these bubbles expand and the loaf grows in volume. As long as the yeast is actively fermenting sugars, the bubbles expand, but as the sugars available to the yeast become depleted, the production of gases starts to slow down. Because the bubble walls are semipermeable, some gases are always escaping. When production stops, the bubbles lose pressure, and poking at the dough leaves an indent. This is the stage at which the loaf is ready to be baked. It has expanded as much as it can before it goes into the oven, where the yeast undergoes a last gasp of fermentation and the gases themselves heat up and expand the dough another 10 to 20 percent.