Quite elaborate explanation is here. The original developer shut down his website

https://thebitcoinmanual.com/articles/hosted-lightning-channels/

Also fiat channels are based on this idea.

Also a post about general concepts, less specific about BLW or SBW wallets which implemented hosted channels since 2019.

https://medium.com/coinmonks/liquidity-abstraction-in-lightning-network-3d7a1d76ac82