Expansion is the invisible movement that is the last action before the arrow is released. The arrow slides under the clicker.
The expansion is not a visible movement of anything, but is a continuation of moving the LAN2 to the spine with an opening of the chest. Expansion should last no more than 2 to 3 seconds. To have expansion occur, just continue to the pull LAN2 to the spine and allow your chest to relax so it opens.
You can simulate expansion by trying this: