As far as I understood, the first order effect is that excluding Russia from swift disables the usual way for Germany to pay for Russian gas and oil, which means that unless they physically move a truckload of euros to Moscow (and get Moscow to agree to that) they are cutting themselves off from one of their major energy supplier. Plus Russia is a big trade partner, but any outstanding bills will then likely go unpaid, and most likely a lot of contracts will be voided.