The preciousblock RPC is used to treat a block as if it were received before other blocks with the same work.

How is this used?

The preciousblock RPC is typically used for testing purposes or in scenarios where a node operator needs to manipulate the order of block reception. By marking a block as precious, a node can prioritize it over others with the same work, effectively altering the blockchain's perceived history within the node's context. This command is handy for simulating various network conditions or verifying the behavior of Bitcoin nodes in different scenarios. However, its effects are temporary and do not persist across node restarts, making it suitable primarily for testing and debugging purposes.