Checkpoint (Anything) by (Anything)

From any appropriate formation: The centers of each side (shown in black below) do Call #1, and, if it ends in a box, there is a mandatory axis change. The remaining outside dancers slide inward, and the new centers do Call #2 and stay in the center.

For example: Checkpoint Ah So by Swing Thru:

Checkpoint Tag the Line by Swing Thru: