Spark is a variant of Statechains.
Statechains are an original second-layer protocol originally developed by Ruben Somsen in 2018.
Statechains are a proposed off-chain system that allows a user (such as Alice) to delegate the ability to spend a UTXO to another user (Bob), who can then further delegate the spending authority to a third user (Carol), and so on.
The off-chain delegation operations are all performed using signature adaptors and the cooperation of a trusted third party who employs the eltoo mechanism—and their knowledge of every previous delegation—to ensure each new delegation uses a state number higher than any previously used state number. These incrementing state numbers ensure that an on-chain spend by the most recent delegate (Carol) can take precedence over spends by previous delegates (Alice and Bob), provided the trusted third party hasn’t colluded with a previous delegate to cheat.
Beyond collusion with a delegated signer (such as Alice or Bob), there is no way for the trusted third party to steal funds. A delegated signer can always spend the UTXO on-chain without needing permission from the trusted third party, arguably making Statechains less trusted than federated sidechains.
We can think of Statechains as a digital version of Opendime. We know that Statechains can’t provide the same trustless self-custody as the Lightning Network, but we still like them. Statechains can be part of the Lightning Network.
https://medium.com/@RubenSomsen/statechains-non-custodial-off-chain-bitcoin-transfer-1ae4845a4a39
https://bitcoinops.org/en/topics/statechains/
https://bitcoinmagazine.com/technical/bitcoin-layer-2-statechains
