2 Answers

The let down stage only occurs if something went wrong with your dough.
# 5 in these different dough stages.
1. pick up : dough is sticky, cold and lumpy
2. initial development : dough is getting warmer, smoother and drier
3. clean up : dough is at maximum stiffness and comes together as one mass. The color will also change from yellowish to more white
4. final development : dough is at its correct temperature and handling quality. A gluten film can be easily obtained by extending a piece of dough
5. letdown : dough is too warm and sticky, it lacks elasticity and has too much flow
6. breakdown : dough is beginning to liquefy

I got this and copied it from a Google search of "let down stage - yeast"