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I think that the the system is made so the SBD and STEEM price will never differ more than 20%

What makes you think that? The prices are up to the market

It's in the withepaper, when one excedes that diference the other stops being printed and Blocktrades locks the transactions.

I've not heard that before. Can you point me to where it's written? The only control I'm aware of is to try and keep SBD around US$1. The price of that went crazy for a while, but is back to the peg now. I've never heard of any restriction on Steem. Even if Blocktrades won't trade it others will.