it doesn't break anything. it adds a new transaction type via segwit witness programs. number 3 since idk maybe someone already has in mind to make a 2. the transaction is basically identical to legacy except it uses schnorr signatures and double sha256 hash instead of sha256/ripemd160
the remainder of the fork is simply adding back all the configurations that core has removed, and setting defaults that are in line with BIP-110, and restoring access to them in the GUI
it turns out that anyone can create new segwit based transaction types at all. the only caveat is that only a client that understands the data in the witness - which is the schnorr stuff - can spend it again. all existing nodes will accept it, they just won't let you spend it because they don't have the code to verify the payment and generate the new transactions to spend from it.
