Rather than broadcasting to every node, the transaction goes to a relay that passes it to builders. It appears on-chain only once included, which removes the window in which a sandwich-attack could be constructed around it.
Several wallets and RPC endpoints offer this by default, sometimes rebating part of the MEV that would otherwise have been extracted. It is the single most effective retail defence for large swaps, and it costs nothing beyond changing an RPC setting.
The trade-offs are modest but real: you are trusting the relay not to leak or exploit your order, inclusion can be slower if few builders receive it, and a failed-transaction may surface differently. It also does nothing about price-impact, which is your own order's doing.
Related: mev, sandwich-attack, mempool, slippage-tolerance