Pretty similar to how lightning does it: each hop between you and your destination forwards an htlc. Like with keysend, you send the secret to the destination out of band. Like with zaplocker, he redeems his htlc when he gets online. Liquidity is locked up along the route until then, annoying every routing node severely. (I hope they charge massive fees, and I hope there are as few routing nodes as possible -- ideally just one, see the Burrow idea at the bottom of the readme.)

It's an essential requirement