For the open stance forehand, look at the initial starting point in the video listed above. If you line up perpendicular to the net like you are about to hit the ball - then drop your front foot to where a line between it and your back foot is parallel to the net and keep your hips and above in the same position, you will feel "wound up" - a rotational stretch from legs to shoulder. Then you release the stored energy into body rotation.
In the case where the ball is fed right into your strikezone, just go ahead and hit it in your neutral stance. You don't need to add complexity to try and get an open stance.