As elsewhere in the booklet, the English text is rather deficient. I'll present it anyway, hoping that some reader can make better sense of it than I can:How to mast or how to make sail differs according to the strength or direction of the wind.
The angle of a mast is adjustable by changing the position of HAIUSHIMI [i.e., mast step]. [Of course, the mast step doesn't move: it's the position of the mast in the step that changes.]
When the wind is not hard , the mast is set up vertically as shown in Fig. B and when the wind is hard, the mast is inclined towards the stern as shown in Fig. C, sometimes with the sail taken in a reef by one FUZAN's [i.e., batten's] length.
Since the way to mast SABANI is not complicated it can easily meet the change of the winds. Quartering (MASUBI) is easiest for SABANI to sail, with its sail open as shown in Fig. D.
Sailing with wind abeam or quatering (sic) is called USAGIBAI or USAGIBARASHI, then a skipper sits in the middle to draw the boat deep so as not to be driven sideways by the wind, as is shown in Fig. E.
Whew! Hard to make sense of that, aside from the interesting point about the skipper moving forward to try to increase the lateral plane when the wind's abeam. Considering that there's no built-in lateral plane (leeboard, keel, outrigger float, etc.), one wonders how well sabanis go to windward.
I also question the angling of the mast depending on the strength of the wind. I suspect that the real function of the multi-position mast step is to change the fore-and-aft location of the center of effort, as on a windsurfer, to aid in steering -- i.e., tilt the mast forward when heading downwind, and tilt the mast aft when heading upwind.