Gel phase and insulation
Updated
The heat the reaction generates, and the decision about whether to let it run.
What gel is
Saponification is exothermic. In a mass of soap the heat builds and the soap passes through a translucent, gel-like phase, starting in the middle and spreading out.
It is not a fault and it is not required. It is a state the soap can pass through, and whether it does changes the finished bar.
Gel or not is a choice
Gelled: deeper, more vivid color; a slightly harder bar sooner; a more translucent look; and it can be unmolded earlier.
Ungelled: paler, more pastel, a more matt opaque finish, and a longer wait before unmolding.
Neither is better. What matters is choosing one and getting it consistently, because the failure is a batch that gelled in the middle and not at the edges.
Forcing gel
Insulate: a lid, a towel or a blanket over the mold, or a warm cupboard.
Some makers put the mold into a low oven briefly and then turn it off.
Watch for overheating, which is the other side of this: cracking down the middle, a volcano erupting from the top, or a soft overheated patch. If the top is cracking, uncover it immediately.
Preventing it
Do not insulate. Pour at a lower temperature. Use a mold that loses heat, and give it airflow.
Refrigerate or freeze the mold for the first several hours, which is the reliable method for milk and honey soaps.
And note the interaction with additives: sugars generate additional heat, so a milk soap will gel and overheat unless actively prevented, which is why those batches are made cold and uninsulated.