The 'force open' function on the trading platform allows you to enter a new bet in the opposite direction to an existing bet on the same market.
This will ensure you have two distinct positions open, except in the case where you have a short position on an unborrowable share. In such cases, the two positions will not be treated as 'distinct' as you will not be able to close the long position unless you close the short position first. Closing the long position ahead of the short will be treated in the same way as opening a new short position which you cannot do on an unborrowable share.
If you do not check 'force open', all or some of your existing position may be closed.
When using IG's desktop platform, the ability to 'force open' is listed underneath the size field. If left unchecked then our platform will close any existing positions that would be cancelled out by the new position (unless you attach a stop or a limit, in which case it will keep both positions open). If you buy 100 shares of Exxon Mobile when you have an open position to sell 100 shares of Exxon Mobile, for instance, IG would simply close the open position: unless you add a stop or a limit.
If the 'force open' box is checked, our platform will not do this and the position will be opened as a separate, new trade. In this respect, a forced open is a different matter to a forced close, which occurs when a position is closed automatically without any action by a trader.