It is common to let go of the belly and/or the butt, especially once you have your arms overhead. It takes concentration and dedication to squeeze it all down in a position that challenges your shoulder flexibility. But it is critical to your safety, your efficiency, and your long-term improvement. Lock it down, hold it in and it will pay off.
Five rounds for time of:
Run 400 meters
20 Overhead walking lunge steps
2 Rope climb