When central banks peg their currencies to others, that means they buy the foreign currency if their own currency is overvalued in comparison, or sell foreign currency to buy local curerncy in the case of undervaluation.
You say that a peg breaks if the Central Bank doesn't have the exchange reserves anymore to uphold a peg.
Where does Bitfinex take the money from to buy tether to prevent it from devaluing? Alternatively, who else buys tether, arbitrageurs?
