A few tips to help master the butterfly weave:

This weave is based off of the thread the needle, so it will help to know that beforehand.

In the video, I am in a normal butterfly rotation on the left side.
To start ( in normal time, not split time, butterfly on left :

1.) Bring the right hand over the left, while letting the left string start to move to the right side

2.) let the right hand slide along the left forearm to the other side, bringing the RH up and under the left forearm while on the right side.

3.) Let the right string start to move to the left side, and start to slide the left wrist on the right forearm back to the left side.

4.) Bring the sticks back into a normal butterfly, or continue the weave by repeating 1