Use One-Way Mode for straightforward directional trades where your view on a contract is a single long or short. Use Hedge Mode when you need offsetting or independent positions on the same contract.
Traders use Hedge Mode to protect a core position through short-term volatility without closing the underlying trade. It can also be used for funding rate arbitrage, holding a long spot position against a short perpetual leg to capture funding payments. Hedge Mode also supports multi-strategy accounts, where a tactical scalp keeps its own PnL line separate from a longer-held directional trade.
On BitMEX, Switch Mode lets you move between the two without closing open positions, so the choice is no longer a commitment.