Just-in-time channel
A just-in-time channel (JIT channel) is a Lightning Network channel that a Lightning service provider (LSP) opens in response to an incoming payment for a client that does not yet have inbound capacity. The LSP holds the first payment, funds a new channel, then forwards the HTLC into that channel so a new user can receive without first opening a channel or buying inbound liquidity.
JIT channels should not be confused with just-in-time routing, which rebalances existing channels so a payment that would otherwise fail for lack of local liquidity can be accepted.
Receiving on Lightning needs inbound capacity: a channel where the remote side holds enough balance (and a free HTLC slot) to carry the payment. A new wallet has neither. A JIT open treats the incoming payment as the event that creates the channel. Negotiation is commonly specified by LSPS2 (bLIP-52). The open usually depends on zero-confirmation channel features (option_zeroconf and option_scid_alias) so peers can use a short-channel-id alias before the funding transaction confirms. See also Zero-confirmation transaction.
In outline: the client gets JIT parameters from an LSP and embeds a reserved SCID in invoice routing hints. When a matching payment arrives, the LSP holds the HTLC, opens an often-unannounced channel with the client, funds it (crediting the payment minus any opening fee), and forwards the HTLC so the client can settle with the preimage. Without extra on-chain contracts, the first open typically requires trusting the LSP for that handshake. After a channel exists, later capacity changes are often done with Splicing rather than more JIT opens. Related liquidity tooling includes Dual funding.