We're going to make another diagram to explain this better with visuals. The key is things are being sent to temporary addresses. I'll come up with something better soon.
Think of it like this.
You -> PosA Node #1 -> Receiver (temp address) -> PoSA Node #2 -> Actual Receiver Address
PoSa Node #1 and #2 knows its safe to send to the Receiver temp address because of block escrow, a new transaction in the blockchain that is verified by all peers just like how normal transactions are.
It's ANON because PoSA Node #2 uses different 'cloak' then what he received to send money to you breaking the trace.
You could think of it like this.
Bob has $50, wants to send $50 to alice. All his dollars are tainted with the letter B. If you send the money to alice you can see the B's and you know it's from Bob.
If the money from PoSA Node#2 came from someone else (different traces) and was sent to the receiver it's Anon. Essentially Alice see's money with different letters on it, say C or something.
This is what is happening in the final steps, this does require that the PoSA Nodes actually have the money to send so balance checks would be in place.
Right, if Alice gets her coins from say, Charlie, and the coins have C's on them, this hides the fact that these originated from Bob. But how do you prevent people from following the chain and figuring out who sent coins to Charlie just before he sent some coins to Alice, and finding Bob this way?
PoSA anon breaks the chain that you would expect to be able to follow. You just arrive at a dead end meaning it can't be followed.