The instructions in the soybeans say that parboil first and then simmer slowly for two hours.
Let's consider this in the scale of my plate. The power-scale of my plate is from 1 to 6. I put it first to 2, then to 3 and then to 4. It started to boil little with 4. Therefore, I consider that simmer slowly means no bubbles in the water i.e. 3.
I assume that parboil means "put the power of the plate to 6 and wait that everything bubbles vigorously".