Rusty has explained it in his blog series which can be found on http://rusty.ozlabs.org/?p=477. Benjamin also asks about the risk to the counterparty and why it would be trustless to route through that party. Rusty explains how they can delay the payment until it times out or force to flush to the blockchain and wait for the timeout. Moreover, Benjamin inquires about how users of the network can find and select those intermediaries. Rusty states that it is an open question, and he is going to have a conversation with CJP (the author of Amiko pay) on the new list about it.