It's pretty simple really no magic involved. Basically as the water level increases past the level of the main drain the water pressure increases pushing more flow through the main drain. In other words, the amount of water flowing through the main drain when the water level is lets say 10 cm above the main drain is more then the flow when the water is only 1 cm above the main drain.
I personally only have two stnand pipes @ 1.5", my main drain with the gate valve fully open is more then enough to handle 1000 GPH without going into full siphon. The noise is horribble though, so I cut it back to about 10% open which causes the water level to be just at my secondary stand pipe. I find that I need to have some flow through the secondary pipe to reduce the noise. The way I figure is that if the water ever goes above the secondary stand pipe it will create a full siphon and will be more then enough capacity.
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