In general, works by reducing the exposure of a trade by closing a part of your open trade. For example, if you have a 0.5 lot trade on GBPAUD, you can partially close this trade by closing 0.1 lots. A new trade will then be created for the remaining 0.4 lots, and the existing trade will turn to a volume of 0.1. Both your partially closed trade and the newly opened trade will have the same open price, as they were opened at the same time. The 0.1 lot trade will then have whatever market price was live when the trade was closed and of course the 0.4 lot trade which remains open will have a different close price based on what is live at the time.
This concept, however, does not work nor is it available on the open of a trade. If you open a 0.5 lot trade, you cannot simply add volume to this trade. This would mean if the market moved in your favour, you could increase volume and receive higher exposure on an open price you know is beneficial. Instead, if you wish to increase your exposure what you should do is simply open a new trade and it will be opened at the live market price, both trades will then continue to be affected by the same live market price, but the price they were opened on is different.