This is simply a mechanism that protects the sender from sending the wrong type of currency, incorrect amount, or incorrect address used. It is a simple hold placed on the pending transfer. That is it. details of the pending transaction would show up in the deposit wallet. Think of it like a memo that sends 0.001 STEEM. It would send a memo to the deposit address and include details of the pending transaction.
Sounds like a confirmation mechanism.
What the handshake gives you is a blockchain proof of the seller confirming that the tx is well formed for whatever purpose that means. And I don't see much value in that so far.
But feel free to make a point as to why this is the next killer app.