In this video I show a very basic and powerful sweep/take down from the Butterfly position.
The driving force behind this move is a strong Technical Standup. We practice this every day in our warmups and it applies in a lot more instances than you would think. When you build a good foundation for movement it can really help your grappling.
This is one of the first videos that I introduce the importance of head control in Butterfly Guard. The person that has the head placement is going to control the position (top OR bottom). So it is very important to offset your body by slightly shrimping to one side and placing the top of your head, near the forehead, on the side of the neck.
The biggest problem that I see with this move is that people bend their neck to get the inside track. This actually does the opposite of what you want. Make sure that your neck is straight otherwise it will cut off the power to your hips and you will get flattened out and crushed.
While performing the Technical Standup it is important to make use of the space behind you rather than trying to push into your opponent. It is important to set up your position BEFORE you try to drive your opponent over.
If you have good head placement and good base it should be fairly easy to overtake your opponent when driving forward. Reference the video so that you can see that it is not that important to control that arm (at least not as important as head placement).