From a closed position
The closed position is the most commonly used position. If you change it to an open position, most opponents will immediately change it back to closed position. Therefore, the open position usually exists for only a few seconds at most. To switch to a closed position with the other leg in the lead, switch to a open position and the opponent will usually close the position so that the opposite sides are now leading.
If in a closed position and your trailing leg is your higher kicking leg, a quick change into an open position will put that leg in the lead and give you a few seconds to fire it to the back of the opponent’s head. If an opponent has a favorite kicking leg, opening or closing a stance will hinder the opponent’s use of that leg, at least for a few seconds. Since you know that, if you open a stance, the opponent will probably close it, you can prepare for the opponent’s stance shift and attack the opponent in mid shift.