The weak subjectivity period refers to how far behind the chain head a node can be before 1/3 of validators may have exited since the node was last in sync.
For example, if 1/3 of validators withdraw their stake and continue signing blocks and attestations, they can form a chain which conflicts with the finalized state. If your node is far enough behind the chain head not to be aware that they've withdrawn their funds, these validators can act dishonestly and continue feeding you blocks to lead you down the wrong chain.
If a node is aware that a validator has withdrawn its funds, the node will reject the validator's attestations.
Safely sync your node
Teku provides two methods to safely sync a node that is new to the network or has been offline for an extended period.
--initial-stateto supply an SSZ encoded state file from which to sync.
--ws-checkpointto supply a weak subjectivity checkpoint by which a node can securely validate its view of the current state.
We recommend using
--initial-state. It provides the same security benefits as
--ws-checkpoint, but syncs faster. The only exception is when syncing an archive node, in which case, use
/eth/v2/debug/beacon/states/<state_id> API on an updated node to download a recent finalized state as an SSZ encoded state file.
Another option is to reconstruct historical states. This allows the creation of a full archive node, ensuring that once the node is up-to-date, the concerns associated with weak subjectivity are cleared.