JeroenE The isolated account contains all the money, when a payment is initiated, it sends the total amount from that account to the public account, from which the money is transferred. This account then has a € 0 balance again. The usefulness here is a bit more limited in the Netherlands, I am used to it from the US because direct debits there are rather poopy and unsafe. But I still like the idea that money is on an IBAN that is never shared with others (except for the tax authorities, but that's okay).
The payment would not necessarily be initiated from the bunq app, but it would actually be nice if the bunq app itself could also have these types of rulesets. Incoming payment to the escrow account? Immediately remove it and move it to the private account.