Force open vs net off
Force open enables you to maintain your exposure to a market while opening a new position in the same market, which can be a useful tool for hedging.
Without the force open function, most platforms will net off your positions. In a netted off trade, an existing position would be automatically closed if the new position would cancel it out (unless you attach a stop or a limit, in which case it will keep both positions open).
Placing two trades of the same value using the force open function will create a net exposure of zero – the same as having no open positions. However, you would still pay any applicable overnight funding or spread, which is something to be aware of when using the force open feature.