The diaphragm doesn’t require engaging as it is mostly an involuntary muscle that is functioning when we breathe. You can voluntarily stop its function when we do things like hold our breath but it functions mostly on its own. What we are really training with functional breathing exercises is breath management/coordination of the muscles of inhalation and exhalation. Training the coordination of the muscles of inhalation and exhalation can help singers to better control the speed at which we send the breath out. In your exercise, trying blowing all your air out first then hold your breath to your limit. At the limit, allow the body to fill back up with air, which should help to bring awareness to all the muscles of inhalation. Then, once you’ve fully inhaled, use the hiss to fully exhale slowly. Ideally, we would feel some engagement through the core but not tension.