Hedge Mode itself is standard across major derivatives venues. The difference on BitMEX is the switch support. On other major exchanges, traders are required to close all open positions before changing their position mode. BitMEX carries open positions over with entry price, PnL, and margin intact. Traders still need to cancel active orders before the switch.
A trader holding a 10 BTC long on BitMEX can move from One-Way Mode to Hedge Mode without closing the position, cancelling the PnL, or paying exit and re-entry fees. The position carries over into the new mode intact. Switch Mode closes a workflow gap that has been present across the rest of the derivatives market.