How do Sphinx payments with no bill work in the Lightning network?

Recently a WIP draft has been published Ind Github, for a & # 39; sphinx payment mode & # 39; for Lightning network nodes. To quote Roasbeef from Github PR:

[this allows] the ability to send a payment to a destination without the need for an invoice

Although this is a WIP, it seems that it can already be used for Mainnet, as long as all affected nodes are updated with the new code.

How does this work? What does the UX look like at a high level? What are the differences at a low level in terms of completing payment (HTLCs all around) compared to a standard LN transaction? What's going on under the hood?