It is perhaps the most repeated phrase in vocal training: "You need to sing from your diaphragm." We are often told to breathe into it, "pop" it, or "push" it to control our breath. But the reality of human biology tells a very different story about how we actually produce sound.
To put it simply, the diaphragm is not the central focus of learning how to sing. In fact, it isn't something a singer can directly control at all. The diaphragm is an involuntary muscle that works automatically during the respiratory process.
When your body signals that it needs oxygen, your lungs inflate. To make room for this expansion, the diaphragm—which is shaped like an inverted bowl—flattens out. As you exhale, it naturally returns to its dome shape. This entire process happens without you ever giving it a single command. You cannot "tell" your diaphragm to flatten or rise any more than you can tell your heart to beat.
If you can't control the diaphragm, what can you do? The key is to understand your music and text. When you have a clear mental goal for the phrase you are about to sing, your body—including your respiratory system—will respond naturally as it was designed to do.
While breathing is vital for singing, it is actually secondary to a more primary process called Registration. Registration is the way your vocal muscles (the "stretchers" and "closers") adjust your vocal cords for pitch and volume. If your registration is out of balance, no amount of "diaphragm control" will fix the sound.
Stop worrying about "singing from the diaphragm." It has been doing its job perfectly since the day you were born. Instead, focus on developing your registration and interpreting your music. When the conditions are right, your breathing will take care of itself, leaving you free to focus on the art of singing.