The board can be flipped over after the deck and several rail strips on each side have been installed. The board will be stiff enough to maintain its rocker profile at this point. The photo below shows the rocker tabs (board shown without strips for clarity) which extend from the bottom of the frame to the flat assembly table.
To release the board from the assembly table and get it ready to flip, reach under the board with a pull saw and cut the rocker tabs loose, or, if clamps were used to attach the rocker tabs to the assembly blocks, the clamps can be removed. The board in the photo below, with the addition of several rail strips on each side, is ready to be flipped over.
When the board has been released from the assembly table it is flipped over and secured to temporary blocking. A small patch of wood glue can be used to make this temporary attachment.
The board in this image has been flipped, secured to the assembly table the rocker tabs have been cut and blocked planed flush with the bottom. The supplemental gluing strips were added after the rocker tabs were block planed flush.
For more on flipping the board check out this:
There are many ways to learn to safely use the tools and execute the processes necessary to build your hollow board. But the bottom line is this: Tools and the dust they create are dangerous. If you are a novice woodworker, educate yourself about tool safety and proper shop practices. Power tools are very unforgiving and trouble can happen quickly if you are not prepared and diligent in your approach to how you use these tools. Regarding the health consequences of breathing wood dust, take this issue seriously. Use good respirators with fresh, clean filters when cutting or sanding wood. This is also true for applying and sanding epoxy and other finish products. Uncured epoxy is toxic. Most of the ultra violet inhibiting finishes that go over the epoxy for the final finish are also toxic. Use the tools and materials at your own risk. Educate yourself and work smart and safe!