Goods sold in the SOI are sold simultaneously.
Assume there is production of 50 in your country, and 150 in your Chinese sphereling.
Suppose there is demand of 100 in your country and 100 in your Chinese sphereling. Everyone sells everything they produce and buys everything they want.
Now suppose demand crashes to half: 50 in your country and 50 in China. You both sell half of what you produce: you sell 25 and China sells 75, and the remaining 100 gets offered to the Rest of the World, and dumped if they don't want it.
Now suppose demand doubled: 200 in your country and 200 in China. Since there is excess demand, you get first grab. The Chinese get to sell all their stuff (to you), but they don't get to buy anything, because you have grabbed the lot.
Sphering gives an advantage in consumption of scarce resources, and a disadvantage in production of common resources. Early on, hordes of Chinese artisans can turn almost anything they can make common and your fledgling industry will be stuffed by them. You need a tech advantage (so your industry can stuff them and they give up and make something else instead) or access to a raw material they don't have (so you can use being ahead in the purchasing queue to stop them making it at all).