With this service enabled, you keep the Ethereum chain data in swarm, so your ethereum node can function as a flexible light client.
Sword leverages the new chain and state access layers to the core, to provide an on-demand retrieval plugin. Perfectly identical to the LES-based light client, SWORD provides integrity protected (merkle proof verified) on-demand access to the ethereum blockchain data. Chain data include blocks, transactions (maybe even block headers), receipts, state and contract storage. This means a fast user-experience that allows you to do anything a full archival node can without major compromise to security yet without ever running the VM or storing any particular part of the chain and state database.
SWORD is completely resistant to local storage or memory failures but requires network connectivity and a healthy swarm network with sufficient provision (enough nodes submitting, storing and serving chain data). On the one hand a